cmake/boost.cmake: fixes search path for (linked) boost package if bu…

…ild in sub-directory (take a look into `include/boost` before `include`);

otherwise `cd build/Release_x64; cmake -S ../.. -B .` may fail, because it cannot find linked boost folder.

cmake/boost.cmake

@@ -13,15 +13,19 @@ set(BOOST_NO_BOOST_CMAKE ON)
 
 unset(Boost_INCLUDE_DIR CACHE)
 # we might have boost in tree, so provide a hint and try again
-set(BOOST_INCLUDEDIR "${PROJECT_SOURCE_DIR}/include")
+set(BOOST_INCLUDEDIR "${PROJECT_SOURCE_DIR}/include/boost")
 find_package(Boost ${BOOST_MINVERSION} QUIET)
 if(NOT Boost_FOUND)
+  set(BOOST_INCLUDEDIR "${PROJECT_SOURCE_DIR}/include")
+  find_package(Boost ${BOOST_MINVERSION} QUIET)
+  if(NOT Boost_FOUND)
     # otherwise check for Boost installed on the system
     unset(BOOST_INCLUDEDIR)
     find_package(Boost ${BOOST_MINVERSION} QUIET)
     if(NOT Boost_FOUND)
         message(FATAL_ERROR "Boost ${BOOST_MINVERSION} or later not found. Either install system packages if available, extract Boost headers to ${CMAKE_SOURCE_DIR}/include, or set the CMake BOOST_ROOT variable.")
     endif()
+  endif()
 endif()
 
 message(STATUS "Boost version: ${Boost_MAJOR_VERSION}.${Boost_MINOR_VERSION}.${Boost_SUBMINOR_VERSION}")