oeqa/selftest/buildoptions: Improve ccache test

This test occisionally fails as m4 doesn't recompile, meaning the logfile test
then doesn't find mention of ccache.

To ensure m4 does recompile, clean m4 before force compiling it.

(Reading the test is confusing due to the test cleanup also involving a clean)

(From OE-Core rev: 6e0b9214a0d57ed45a5df0ba5c9887a9045b89b1)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oeqa/selftest/cases/buildoptions.py

@@ -38,6 +38,7 @@ class ImageOptionsTests(OESelftestTestCase):
         self.assertTrue(os.path.isfile(p), msg = "No ccache found (%s)" % p)
         self.write_config('INHERIT += "ccache"')
         self.add_command_to_tearDown('bitbake -c clean m4')
+        bitbake("m4 -c clean")
         bitbake("m4 -f -c compile")
         log_compile = os.path.join(get_bb_var("WORKDIR","m4"), "temp/log.do_compile")
         with open(log_compile, "r") as f: