Use clang as the default C compiler

Clang compiles faster than GCC and is supposed to give more meaningful
error messages, so let's use it at least for development.

OwlPS Positioner does not compile with clang++, so we are stuck with g++
(or colorgcc).

common.mk

@@ -53,9 +53,14 @@ SYMLINK := ln -sf
 
 # C compiler
 ifeq "$(origin CC)" "default"
-	COLORGCC := $(shell which colorgcc >/dev/null 2>&1 ; echo $$?)
-	ifeq "$(COLORGCC)" "0"
-		CC := colorgcc
+	CLANG := $(shell command -v clang >/dev/null ; echo $$?)
+	ifeq "$(CLANG)" "0"
+		CC := clang
+	else
+		COLORGCC := $(shell command -v colorgcc >/dev/null ; echo $$?)
+		ifeq "$(COLORGCC)" "0"
+			CC := colorgcc
+		endif
 	endif
 endif