Fix CLT detection on Mavericks/Xcode5

On Mavericks/Xcode5 machines the Command Line Tools are shipped directly
with Xcode. Detect the corret path to the tools in this case.

cc is just a symlink to clang so use gcc as toolname.
The include files are in the SDK path now. Get this path using xcrun.

Signed-off-by: Christoph Kassen <christoph@kassen.name>