Make it possible to compile chromium-efl on desktop using clang

Make it possible to compile chromium-efl on desktop using clang

Chromium provided version of clang has certain adventages over GCC:
* The compiler checks the code for coding style violations. It treats
  them as warnings.
* Diagnostic messages displayed by the compiler are much more readable.
  They use colors to signify the type of problem (red - error, yellow -
  warning, etc), add custom ASCI art like indicators showing where the
  problem is, display tips how the problem can be fixed.
* Chromium.org has switched to clang as the default compiler some time
  around M38.

This patch makes it easy to build chromium-efl using clang. It's enough
to pass --clang flag to build_desktop.sh. The feature uses clang version
shipped together with chromium sources so the user does not need to
install anything on his/her system.
Reviewed by: Antonio Gomes, SeungSeop Park (OOO ~ 4/Jan/2015), Viatcheslav Ostapenko
Reviewed by: Antonio Gomes, Daniel Waślicki, Kamil Klimek, SeungSeop Park

Change-Id: I2e3966098aa397b6712c761709880ed9a5fa5294
Signed-off-by: Piotr Tworek <p.tworek@samsung.com>