Fix for thunderbird binary build error: cc not found?

Hey,

So mail/thunderbird if you try to build it from pkgsrc,  you get an 
error saying cc does not exist.
This could be a fix for that, i got further but missed dependencies, but 
only because i have not built it before... i suppose.

patches/patch-comm_third__party_openpgp.configure
-                "--cc={}".format(os.getenv("CC"))
+                "--cc={}".format(if os.getenv("CC") else import shutil; 
shutil.
which("cc")),

br Nicklas