cmake: Only check for bounded attribute on OpenBSD

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>

ConfigureChecks.cmake

@@ -370,6 +370,8 @@ int main(void) {
     return 0;
 }" HAVE_COMPILER__FUNCTION__)
 
+# This is only available with OpenBSD's gcc implementation */
+if (OPENBSD)
 check_c_source_compiles("
 #define ARRAY_LEN 16
 void test_attr(const unsigned char *k)
@@ -378,6 +380,7 @@ void test_attr(const unsigned char *k)
 int main(void) {
     return 0;
 }" HAVE_GCC_BOUNDED_ATTRIBUTE)
+endif(OPENBSD)
 
 # Stop treating warnings as errors
 unset(CMAKE_REQUIRED_FLAGS)