Check for gcc before setting gcc specific CFLAGS

configure.ac

@@ -9,6 +9,7 @@ AC_CONFIG_HEADERS([config.h])
 # Checks for programs.
 AC_PROG_CC
 AC_PROG_LIBTOOL
+AM_CONDITIONAL([GCC], [test x$GCC = xyes])
 
 # Checks for libraries.
 

src/Makefile.am

@@ -17,4 +17,7 @@ libjansson_la_LDFLAGS = \
 	-export-symbols-regex '^json_' \
 	-version-info 3:0:3
 
+if GCC
+# These flags are gcc specific
 AM_CFLAGS = -Wall -Wextra -Werror
+endif