compiler: support for custom flags

[if custom]fn..{} , #flag custom, $if custom {}

vlib/compiler/cc.v

@@ -486,24 +486,47 @@ fn find_c_compiler_default() string {
 
 fn find_c_compiler_thirdparty_options() string {
 	fullargs := env_vflags_and_os_args()
-	mut cflags := get_cmdline_cflags(fullargs)
+	mut cflags := get_cmdline_multiple_values(fullargs,'-cflags')
 	$if !windows {
-		cflags += ' -fPIC'
+		cflags << '-fPIC'
 	}
 	if '-m32' in fullargs {
-		cflags += ' -m32'
+		cflags << '-m32'
 	}
-	return cflags
+	return cflags.join(' ')
 }
 
-fn get_cmdline_cflags(args []string) string {
-	mut cflags := ''
+fn get_cmdline_multiple_values(args []string, optname string) []string {
+	mut flags := []string
 	for ci, cv in args {
-		if cv == '-cflags' {
-			cflags += args[ci + 1] + ' '
+		if cv == optname {
+			flags << args[ci + 1]
 		}
 	}
-	return cflags
+	return flags
+}
+
+fn parse_defines(defines []string) ([]string,[]string) {
+	// '-d abc -d xyz=1 -d qwe=0' should produce:
+	// compile_defines:      ['abc','xyz']
+	// compile_defines_all   ['abc','xyz','qwe']
+	mut compile_defines := []string
+	mut compile_defines_all := []string
+	for dfn in defines {
+		dfn_parts := dfn.split('=')
+		if dfn_parts.len == 1 {
+			compile_defines << dfn
+			compile_defines_all << dfn
+			continue
+		}
+		if dfn_parts.len == 2 {
+			compile_defines_all << dfn_parts[0]
+			if dfn_parts[1] == '1' {
+				compile_defines << dfn_parts[0]
+			}
+		}
+	}
+	return compile_defines, compile_defines_all
 }
 
 fn missing_compiler_info() string {