runtests.py: Add support for ccache

runtests.py

@@ -203,6 +203,7 @@ KEEP_2X_FILES = [
 COMPILER = None
 INCLUDE_DIRS = [ d for d in os.getenv('INCLUDE', '').split(os.pathsep) if d ]
 CFLAGS = os.getenv('CFLAGS', '').split()
+CCACHE = os.getenv('CYTHON_RUNTESTS_CCACHE', '').split()
 
 BACKENDS = ['c', 'cpp']
 
@@ -242,13 +243,15 @@ list_unchanging_dir = memoize(lambda x: os.listdir(x))
 
 class build_ext(_build_ext):
     def build_extension(self, ext):
-        if ext.language == 'c++':
         try:
             try: # Py2.7+ & Py3.2+
                 compiler_obj = self.compiler_obj
             except AttributeError:
                 compiler_obj = self.compiler
+            if ext.language == 'c++':
                 compiler_obj.compiler_so.remove('-Wstrict-prototypes')
+            if CCACHE:
+                compiler_obj.compiler_so = CCACHE + compiler_obj.compiler_so
         except Exception:
             pass
         _build_ext.build_extension(self, ext)