--- src/Makefile.in.orig	2018-10-31 17:01:43 UTC
+++ src/Makefile.in
@@ -17,7 +17,7 @@ DESTDIR  = 
 # These are set by configure
 # NB: cc and various flags must match the ones used to compile Gauche,
 # so the make invocator shouldn't casually override them.
-CFLAGS   = @DEFS@ @X_CFLAGS@ @GLUT_INCDIR@ @GLEW_INCDIR@
+CFLAGS   += @DEFS@ @X_CFLAGS@ @GLUT_INCDIR@ @GLEW_INCDIR@
 LDFLAGS  = @LDFLAGS@
 OBJEXT   = @OBJEXT@
 SOEXT    = @SOEXT@
