fix QMAKE_COMPILER fallback for clang specs

in the actual specs, we also set 'gcc' for clang.

Change-Id: Ifc6b27d56596f34c944205795d665f545d090f80
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>

mkspecs/features/spec_post.prf

@@ -42,7 +42,7 @@ isEmpty(QMAKE_COMPILER) {
     else:*-llvm*: \
         QMAKE_COMPILER = gcc llvm
     else:*-clang*: \
-        QMAKE_COMPILER = clang llvm
+        QMAKE_COMPILER = gcc clang llvm
     else:*-msvc*: \
         QMAKE_COMPILER = msvc
     else: \