[GTK] WebKit2 testing bot unable to find the build directory

[GTK] WebKit2 testing bot unable to find the build directory
https://bugs.webkit.org/show_bug.cgi?id=90523

Reviewed by Carlos Garcia Campos.

The presence of GNUmakefile was used to determine whether a given
directory was a valid build directory or not. That's not correct for
testing bots as we export only the minimum required stuff to run the
tests.

* gtk/common.py:
(get_build_path.is_valid_build_directory): added an extra check.:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121837 268f45cc-cd09-0410-ab3c-d52691b4dbfc