Add feature extraction tests for gcc_plugin.

This patch seperates the tests into two tools: mageec and gcc_plugin. It adds a
gcc specific feature extraction test.

	* Makefile.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Add gcc_plugin.tests/Makefile and config/site.exp to
	AC_OUTPUT.
	* testsuite/Makefile.am: Set config/site.exp as a configuration file.
	Add gcc_plugin as a second tool to seperate gcc specific test from
	mageec tests.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/config/site.exp.in: New file. Configuration to pass to the
	tests.
	* testsuite/gcc_plugin.tests/Makefile.am: New file.
	* testsuite/gcc_plugin.tests/Makefile.in: New file, generate.
	* testsuite/gcc_plugin.tests/feature-extract.exp: New test file. Checks
	that features are extracted from compiling an empty source file.
	* testsuite/gcc_plugin.tests/main.c: New empty source file.