Add llvm-config --cflags to CFLAGS (#59419)

It's convenient to use `USE_SYSTEM_LLVM=1` and `LLVM_CONFIG` to build
Julia with a local LLVM build, but the build fails because
`<llvm/Config/llvm-config.h>` is not found when it is not installed into
`build_includedir`. Ask `llvm-config` for the `CFLAGS` to fix this.

Author: xal-0

src/Makefile

@@ -139,6 +139,7 @@ PUBLIC_HEADER_TARGETS := $(addprefix $(build_includedir)/julia/,$(notdir $(PUBLI
 
 LLVM_LDFLAGS := $(shell $(LLVM_CONFIG_HOST) --ldflags)
 LLVM_CXXFLAGS := $(shell $(LLVM_CONFIG_HOST) --cxxflags)
+LLVM_CFLAGS := $(shell $(LLVM_CONFIG_HOST) --cflags)
 
 ifeq ($(OS)_$(BINARY),WINNT_32)
 LLVM_CXXFLAGS += -I$(SRCDIR)/support/win32-clang-ABI-bug
@@ -284,9 +285,9 @@ $(BUILDDIR)/jl_internal_funcs.inc: $(SRCDIR)/jl_exported_funcs.inc
 
 # source file rules
 $(BUILDDIR)/%.o: $(SRCDIR)/%.c $(HEADERS) | $(BUILDDIR)
-	@$(call PRINT_CC, $(CC) $(JCPPFLAGS) $(JCFLAGS) $(JL_CFLAGS) $(SHIPFLAGS) $(DISABLE_ASSERTIONS) -c $< -o $@)
+	@$(call PRINT_CC, $(CC) $(LLVM_CFLAGS) $(JCPPFLAGS) $(JCFLAGS) $(JL_CFLAGS) $(SHIPFLAGS) $(DISABLE_ASSERTIONS) -c $< -o $@)
 $(BUILDDIR)/%.dbg.obj: $(SRCDIR)/%.c $(HEADERS) | $(BUILDDIR)
-	@$(call PRINT_CC, $(CC) $(JCPPFLAGS) $(JCFLAGS) $(JL_CFLAGS) $(DEBUGFLAGS) -c $< -o $@)
+	@$(call PRINT_CC, $(CC) $(LLVM_CFLAGS) $(JCPPFLAGS) $(JCFLAGS) $(JL_CFLAGS) $(DEBUGFLAGS) -c $< -o $@)
 $(BUILDDIR)/%.o: $(SRCDIR)/%.cpp $(SRCDIR)/llvm-version.h $(HEADERS) $(LLVM_CONFIG_ABSOLUTE) | $(BUILDDIR)
 	@$(call PRINT_CC, $(CXX) $(LLVM_CXXFLAGS) $(JCPPFLAGS) $(JCXXFLAGS) $(JL_CXXFLAGS) $(SHIPFLAGS) $(CXX_DISABLE_ASSERTION) -c $< -o $@)
 $(BUILDDIR)/%.dbg.obj: $(SRCDIR)/%.cpp $(SRCDIR)/llvm-version.h $(HEADERS) $(LLVM_CONFIG_ABSOLUTE) | $(BUILDDIR)