diff --git a/Makefile b/Makefile
index 09b21541e..6d37d55b4 100644
--- a/Makefile
+++ b/Makefile
@@ -30,7 +30,7 @@ INSTALL = install
 DIFF    = diff
 PREFIX  ?= /usr/local
 MANDIR  ?= $(PREFIX)/share/man
-CFLAGS  ?= -Os -Wall -Wextra -Wno-unused-parameter -Wshadow -Wmissing-prototypes -Wwrite-strings
+CFLAGS  ?= -std=c99 -D_DEFAULT_SOURCE -D_POSIX_C_SOURCE=200809L -D_BSD_SOURCE -Os -Wall -Wextra -Wno-unused-parameter -Wshadow -Wmissing-prototypes -Wwrite-strings
 EXPORTDIR ?= .
 RANLIB  ?= ranlib
 PKG_CONFIG ?= pkg-config
diff --git a/dmi.c b/dmi.c
index c44221c70..3b717cd23 100644
--- a/dmi.c
+++ b/dmi.c
@@ -19,7 +19,9 @@
 
 /* strnlen is in POSIX but was a GNU extension up to glibc 2.10 */
 #if (__GLIBC__ == 2 && __GLIBC_MINOR__ < 10) || __GLIBC__ < 2
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif /* !GNU_SOURCE */
 #else
 #define _POSIX_C_SOURCE 200809L
 #endif