FFTW: Windows, OMP & Floats

This improves the FFTW CMake find logic based on the scripts that
I developed for HiPACE++/WarpX/ImpactX to find FFTW on various
platforms.

It helps to find FFTW on Windows, where it is installed by default
with CMake, and it uses more details from PkgConfig files on
Linux/macOS, such as forwarding defines, using OpenMP acceleration
if available, customizing floating point variants for SINGLE/DOUBLE
precision builds.

It also chooses sensible defaults to work around known FFTW build
system bugs, linked inline.

Tools/CMake/FindAMReXFFTW.cmake

@@ -9,9 +9,19 @@ Imported Targets
 
 This module provides the following imported target, if found:
 
-``FFTW``
+``AMReX::FFTW``
   The FFTW library
 
+Options/Control Variables
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+``AMReX_FFTW_SEARCH``
+  FFTW search method (PKGCONFIG/CMAKE).
+  Defaults to CMake config packages on Windows and to PkgConfig pc files on Linux/macOS.
+
+``AMReX_FFTW_IGNORE_OMP``
+  Ignore FFTW3 OpenMP support, even if found.
+
 Result Variables
 ^^^^^^^^^^^^^^^^
 
@@ -27,25 +37,110 @@ This will define the following variables:
 This will also create an imported target, AMReX::FFTW.
 #]=======================================================================]
 
-if (NOT FFTW_INCLUDES)
-    find_path(FFTW_INCLUDES NAMES "fftw3.h" HINTS ${FFTW_ROOT}/include)
-endif()
+# Helper Functions ############################################################
+#
+option(AMReX_FFTW_IGNORE_OMP "Ignore FFTW3's OpenMP support, even if found" OFF)
+mark_as_advanced(AMReX_FFTW_IGNORE_OMP)
 
-if (NOT FFTW_LIBRARIES)
-    find_library(FFTW_LIBRARY NAMES "fftw3" HINTS ${FFTW_ROOT}/lib)
-    find_library(FFTWF_LIBRARY NAMES "fftw3f" HINTS ${FFTW_ROOT}/lib)
-    set(FFTW_LIBRARIES ${FFTW_LIBRARY} ${FFTWF_LIBRARY})
-endif()
+# Set the AMReX_FFTW_OMP=1 define on AMReX::FFTW if TRUE and print
+# a message
+#
+function(fftw_add_define HAS_FFTW_OMP_LIB)
+    if(HAS_FFTW_OMP_LIB)
+        message(STATUS "FFTW: Found OpenMP support")
+        target_compile_definitions(AMReX::FFTW INTERFACE AMReX_FFTW_OMP=1)
+    else()
+        message(STATUS "FFTW: Could NOT find OpenMP support")
+    endif()
+endfunction()
 
-include(FindPackageHandleStandardArgs)
+# Check if the found FFTW install location has an _omp library, e.g.,
+# libfftw3(f)_omp.(a|so) shipped and if yes, set the AMReX_FFTW_OMP=1 define.
+#
+function(fftw_check_omp library_paths fftw_precision_suffix)
+    find_library(HAS_FFTW_OMP_LIB fftw3${fftw_precision_suffix}_omp
+            PATHS ${library_paths}
+            # this is intentional, so we don't mix different FFTW installs
+            # and only check what is in the location hinted by the
+            # "library_paths" variable
+            NO_DEFAULT_PATH
+            NO_PACKAGE_ROOT_PATH
+            NO_CMAKE_PATH
+            NO_CMAKE_ENVIRONMENT_PATH
+            NO_SYSTEM_ENVIRONMENT_PATH
+            NO_CMAKE_SYSTEM_PATH
+            NO_CMAKE_FIND_ROOT_PATH
+    )
+    if(HAS_FFTW_OMP_LIB)
+        # the .pc files here forget to link the _omp.a/so files
+        # explicitly - we add those manually to avoid any trouble,
+        # e.g., in static builds.
+        target_link_libraries(AMReX::FFTW INTERFACE ${HAS_FFTW_OMP_LIB})
+    endif()
 
-find_package_handle_standard_args(AMReXFFTW
-    REQUIRED_VARS FFTW_LIBRARIES FFTW_INCLUDES)
+    fftw_add_define("${HAS_FFTW_OMP_LIB}")
+endfunction()
 
-mark_as_advanced(FFTW_LIBRARIES FFTW_INCLUDES)
+
+# Central FFTW3 Search ###############################################
+#
+# On Windows, try searching for FFTW3(f)Config.cmake files first
+#   Installed .pc files wrongly and unconditionally add -lm
+#   https://github.com/FFTW/fftw3/issues/236
+
+# On Linux & macOS, note Autotools install bug:
+#   https://github.com/FFTW/fftw3/issues/235
+# Thus, rely on .pc files
+
+set(AMReX_FFTW_SEARCH_VALUES PKGCONFIG CMAKE)
+set(AMReX_FFTW_SEARCH_DEFAULT PKGCONFIG)
+if(WIN32)
+    set(AMReX_FFTW_SEARCH_DEFAULT CMAKE)
+endif()
+set(AMReX_FFTW_SEARCH ${AMReX_FFTW_SEARCH_DEFAULT}
+        CACHE STRING "FFTW search method (PKGCONFIG/CMAKE)")
+set_property(CACHE AMReX_FFTW_SEARCH PROPERTY STRINGS ${AMReX_FFTW_SEARCH_VALUES})
+if(NOT AMReX_FFTW_SEARCH IN_LIST AMReX_FFTW_SEARCH_VALUES)
+    message(FATAL_ERROR "AMReX_FFTW_SEARCH (${AMReX_FFTW_SEARCH}) must be one of ${AMReX_FFTW_SEARCH_VALUES}")
+endif()
+mark_as_advanced(AMReX_FFTW_SEARCH)
+
+# floating point precision suffixes: float, double and quad precision
+if(AMReX_PRECISION STREQUAL "DOUBLE")
+    set(HFFTWp "")
+else()
+    set(HFFTWp "f")
+endif()
+
+if(AMReX_FFTW_SEARCH STREQUAL CMAKE)
+    find_package(FFTW3${HFFTWp} CONFIG REQUIRED)
+    set(AMReX_FFTW_LIBRARY_DIRS "${FFTW3${HFFTWp}_LIBRARY_DIRS}")
+    message(STATUS "Found FFTW: ${FFTW3${HFFTWp}_DIR} (found version \"${FFTW3${HFFTWp}_VERSION}\")")
+else()
+    find_package(PkgConfig REQUIRED QUIET)
+    pkg_check_modules(fftw3${HFFTWp} REQUIRED IMPORTED_TARGET fftw3${HFFTWp})
+    message(STATUS "Found FFTW: ${fftw3${HFFTWp}_PREFIX}")
+    if(fftw3${HFFTWp}_LIBRARY_DIRS)
+        set(AMReX_FFTW_LIBRARY_DIRS "${fftw3${HFFTWp}_LIBRARY_DIRS}")
+    else()
+        set(AMReX_FFTW_LIBRARY_DIRS "${fftw3${HFFTWp}_LIBDIR}")
+    endif()
+endif()
 
 # Create imported target
 add_library(AMReX::FFTW INTERFACE IMPORTED GLOBAL)
-target_link_libraries(AMReX::FFTW INTERFACE ${FFTW_LIBRARIES})
-set_target_properties(AMReX::FFTW PROPERTIES
-	INTERFACE_INCLUDE_DIRECTORIES "${FFTW_INCLUDES}")
+if(AMReX_FFTW_SEARCH STREQUAL CMAKE)
+    target_link_libraries(AMReX::FFTW INTERFACE FFTW3::fftw3${HFFTWp})
+else()
+    target_link_libraries(AMReX::FFTW INTERFACE PkgConfig::fftw3${HFFTWp})
+endif()
+
+if(AMReX_OMP)
+    if(AMReX_FFTW_IGNORE_OMP)
+        message(STATUS "FFTW: Requested to IGNORE OpenMP support")
+    else()
+        fftw_check_omp("${AMReX_FFTW_LIBRARY_DIRS}" "${HFFTWp}")
+    endif()
+else()
+    message(STATUS "FFTW: Did NOT search for OpenMP support (!AMReX_OMP)")
+endif()