diff --git a/GNUmakefile b/GNUmakefile
index 5a0d6c0e20f37c8aca8d4693ac65cc223ed667ef..d05c0e4451a6759d6fd5c12c1897954950263edf 100644
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -67,12 +67,18 @@ ifeq ($(UNAME_S),Linux)
     OPENMPFLAG=-fopenmp
 endif
 
+# Use gnu99 only on Linux
+# (ideally we would trigger on compiler rather than platform)
+ifeq ($(UNAME_S),Linux)
+    G99FLAG=-std=gnu99
+endif
+
 FEDIR = frontend
-FE_CFLAGS := -g -fPIC -O3 $(OPENMPFLAG) -std=gnu99
+FE_CFLAGS := -g -fPIC -O3 $(OPENMPFLAG) $(G99FLAG)
 FE_CPPFLAGS := $(BASE_CPPFLAGS) -Ithirdparty/sqlite -DGENERICIO_NO_MPI
 
 MPIDIR = mpi
-MPI_CFLAGS := -g -O3 $(OPENMPFLAG) -std=gnu99
+MPI_CFLAGS := -g -O3 $(OPENMPFLAG) $(G99FLAG)
 MPI_CPPFLAGS := $(BASE_CPPFLAGS)
 
 $(FEDIR):