diff --git a/configure.ac b/configure.ac
index 54da101..063bf92 100644
--- a/configure.ac
+++ b/configure.ac
@@ -11,6 +11,9 @@ AM_INIT_AUTOMAKE([1.8 foreign nostdinc])
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])],
                             [AC_SUBST([AM_DEFAULT_VERBOSITY], [1])])
 
+# Check and store if we got user supplied variables
+user_CFLAGS=${CFLAGS-unset}
+
 # Checks for operating system services and capabilities.
 AC_USE_SYSTEM_EXTENSIONS
 AC_SYS_LARGEFILE
@@ -22,6 +25,11 @@ AC_PROG_CC
 AC_PROG_INSTALL
 AC_PROG_LN_S
 
+# Set default compiler variables
+if test "$user_CFLAGS" = unset && test "$GCC" = yes; then
+  CFLAGS="$CFLAGS -Wall -Wextra -Wno-unused-variable -Wno-unused-parameter"
+fi
+
 # Checks for libraries.
 
 # Checks for header files.
diff --git a/src/Makefile.am b/src/Makefile.am
index 4c57136..46b946b 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -1,8 +1,5 @@
 ## Process this file with automake to produce Makefile.in
 
-# Set default values for user variables
-CFLAGS = -Wall -Wextra -Wno-unused-variable -Wno-unused-parameter
-
 AM_CPPFLAGS = \
 	-I$(top_builddir) \
 	-isystem $(top_srcdir)/include/bsd/ \
diff --git a/test/Makefile.am b/test/Makefile.am
index f280136..1d41b86 100644
--- a/test/Makefile.am
+++ b/test/Makefile.am
@@ -1,8 +1,5 @@
 ## Process this file with automake to produce Makefile.in
 
-# Set default values for user variables
-CFLAGS = -Wall -Wextra -Wno-unused-variable -Wno-unused-parameter
-
 AM_CPPFLAGS = \
 	-I$(top_builddir) \
 	-isystem $(top_srcdir)/include/bsd/ \