Append src path to compiler path retrieved from build configuration.

BUG=chromium:369487
TEST=./build/gyp_chromium succeeds in detecting include paths for eclipse generator on chromeos,linux using built in clang.

Review URL: https://codereview.chromium.org/261063005/
Patch from Robert Flack <flackr@chromium.org>!


git-svn-id: http://gyp.googlecode.com/svn/trunk@1912 78cadc50-ecff-11dd-a971-7dbc132099af