Revert "protobuf.pc.in does not reflect CXXFLAGS"

This reverts commit a7180caf19ae150d9a3f9724600b0eed4b5da830.
diff --git a/protobuf.pc.in b/protobuf.pc.in
index 282fef3..4e88b51 100644
--- a/protobuf.pc.in
+++ b/protobuf.pc.in
@@ -8,5 +8,5 @@
 Version: @VERSION@
 Libs: -L${libdir} -lprotobuf @PTHREAD_LIBS@
 Libs.private: @LIBS@
-Cflags: -I${includedir} @PTHREAD_CFLAGS@ @CXXFLAGS@
+Cflags: -I${includedir} @PTHREAD_CFLAGS@
 Conflicts: protobuf-lite