See https://trac.macports.org/ticket/64498
--- a/projects/sample/Makefile.llvm.config.in	2012-11-11 23:11:12.000000000 -0700
+++ b/projects/sample/Makefile.llvm.config.in	2023-02-14 02:43:25.000000000 -0700
@@ -118,6 +118,15 @@
 # Path to the CC binary, which use used by testcases for native builds.
 CC := @CC@
 
+# C/C++ preprocessor flags.
+CPPFLAGS += @CPPFLAGS@
+
+# C compiler flags.
+CFLAGS += @CFLAGS@
+
+# C++ compiler flags.
+CXXFLAGS += @CXXFLAGS@
+
 # Linker flags.
 LDFLAGS+=@LDFLAGS@