ci: fix `clang -std=c99 -D_DEFAULT_SOURCE -o v vc/v.c` warnings

vlib/v/builder/cc.v

@@ -188,7 +188,7 @@ fn (mut v Builder) setup_ccompiler_options(ccompiler string) {
 	// arguments for the C compiler
 	ccoptions.args = [v.pref.cflags]
 	if !v.pref.no_std {
-		ccoptions.args << '-std=c99'
+		ccoptions.args << '-std=c99 -D_DEFAULT_SOURCE'
 	}
 	ccoptions.wargs = [
 		'-Wall',

vlib/v/util/util.v

@@ -519,6 +519,8 @@ pub fn find_all_v_files(roots []string) ?[]string {
 // if you add another cached unsafe function using static, do not forget to add
 // a mechanism to clear its cache, and call it here.
 pub fn free_caches() {
-	unsafe { cached_read_source_file('') or {} }
-	unsafe { cached_file2sourcelines('') }
+	unsafe {
+		cached_file2sourcelines('')
+		cached_read_source_file('') or { '' }
+	}
 }