C++ doesn't grok some C flags

Index: lib/Makefile.in
--- lib/Makefile.in.orig
+++ lib/Makefile.in
@@ -218,6 +218,7 @@ EGREP = @EGREP@
 EXEEXT = @EXEEXT@
 FGREP = @FGREP@
 GENERAL_CFLAGS = @GENERAL_CFLAGS@
+ONLY_CFLAGS = @ONLY_CFLAGS@
 GLUT_CFLAGS = @GLUT_CFLAGS@
 GLUT_LIBS = @GLUT_LIBS@
 GLU_CFLAGS = @GLU_CFLAGS@
@@ -349,10 +350,11 @@ lib_LTLIBRARIES = $(am__append_2) $(am__append_4) $(am
 il_HEADERS = $(all_include)/IL/devil_cpp_wrapper.hpp $(am__append_3) \
 	$(am__append_5) $(am__append_7)
 AM_LDFLAGS = -version-info @DEVIL_LTVERSION@ $(am__append_1)
-AM_CFLAGS = $(GENERAL_CFLAGS)
+AM_CFLAGS = $(ONLY_CFLAGS) $(GENERAL_CFLAGS)
+AM_CXXFLAGS = $(GENERAL_CFLAGS)
 libIL_la_CPPFLAGS = -I $(il_include) -I $(all_include)
-libIL_la_CFLAGS = $(AM_CFLAGS) @IL_CFLAGS@
-libIL_la_CXXFLAGS = $(AM_CFLAGS) @IL_CFLAGS@
+libIL_la_CFLAGS = $(AM_CFLAGS:S/-std=c99//) @IL_CFLAGS@
+libIL_la_CXXFLAGS = $(AM_CXXFLAGS) @IL_CFLAGS@
 libIL_la_SOURCES = $(il_src)/il_alloc.c $(il_src)/il_doom.c \
 	$(il_src)/il_stack.c $(il_src)/il_pcx.c $(il_src)/il_rle.c \
 	$(il_src)/il_bits.c $(il_src)/il_endian.c $(il_src)/il_exr.cpp \