scons: Enable building through Clang Static Analyzer.

scons: Enable building through Clang Static Analyzer.

By accurately detecting gcc/clang through --version option instead
of executable name.

Clang Static Analyzer reports many issues, most false positives, but it
found at least one real and subtle use-after-free issue
in st_texture_get_sampler_view():

  http://people.freedesktop.org/~jrfonseca/scan-build-2014-04-14-1/report-869047.html#EndPath

Reviewed-by: Brian Paul <brianp@vmware.com>