[GTK] Build and run TestWebKitAPI unit tests

[GTK] Build and run TestWebKitAPI unit tests
https://bugs.webkit.org/show_bug.cgi?id=84325

Reviewed by Philippe Normand.

.:

* GNUmakefile.am: Include makefiles to build gtest and
TestWebKitAPI. Add BUILDING_WEBKIT2__ macro to compilation when
building WebKit2.

Source/ThirdParty:

* gtest/GNUmakefile.am: Added.

Tools:

We only build WTF unit tests for now. WebKit2 tests will be
enabled in a follow up patch.

* Scripts/run-gtk-tests:
(TestRunner): Add TestWebKitAPI/WTF to the list of directories
containing unit tests.
(TestRunner._run_test_glib): Run glib tests using gtester.
(TestRunner._run_test_google): Run google tests.
(TestRunner._run_test): Use _run_test_glib or _run_test_google
depending on whether given test is glib or google test
* TestWebKitAPI/GNUmakefile.am: Added.
* TestWebKitAPI/config.h: Use #ifdef instead of # to fix a compile
warning with -Wundef. Include WebKit2 headers only when building
WebKit2.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@115075 268f45cc-cd09-0410-ab3c-d52691b4dbfc