make: export some vars for loader sub-project

This allows to correctly pass variables passed to top-level make, like

make CC=$CC CXX=$CXX

If loader includes clang.mak on its own, then they are replaced
with default values, which is not desirable.

fdpp/clang.mak

@@ -28,6 +28,12 @@ endif
 NASM ?= nasm-segelf
 PKG_CONFIG ?= pkg-config
 
+# export vars needed for loader sub-project
+export CC
+export CC_FOR_BUILD
+export CC_LD
+export PKG_CONFIG
+
 TARGETOPT = -std=c++20 -c -fno-threadsafe-statics -fpic \
   -DCLANG_VER=$(CLANG_VER)
 # _XTRA should go at the end of cmd line

fdpp/loader/makefile

@@ -3,8 +3,11 @@ T = ..
 -include $(T)/local.mak
 srcdir ?= $(CURDIR)
 
+# if exported from top-level project, then don't include defaults
+ifeq ($(PKG_CONFIG),)
 include $(srcdir)/$(T)/clang.mak
 include $(srcdir)/$(T)/defs.mak
+endif
 
 TOP = $(abspath $(srcdir)/$(T)/..)
 HDR = $(TOP)/hdr

fdpp/makefile

@@ -46,6 +46,9 @@ ALL = $(FDPPLIB) $(FDPPDEVL) $(LDRLIB) $(LDRDEVL) $(TARGET).elf $(TARGET).map \
 
 all: $(ALL)
 
+export CFLAGS
+export LDFLAGS
+
 # New makes have a way to avoid parallel invocation with the use of &:
 # On old make you would write "%.elf %.map :" which is a bit silly
 # given the names are fixed. Also $(TARGET).% won't work that way at all: