tests: use ${CC} instead of 'gcc' in tests

tests: use ${CC} instead of 'gcc' in tests

To better support cross-compilation Gentoo provides a way
to configure system without 'gcc' binary and only provide
tool-prefixed tools, like 'x86_64-pc-linux-gnu-gcc'.
The packages are built as ./configure --host=x86_64-pc-linux-gnu.

In https://bugs.gentoo.org/718872 Agostino Sarubbo found
a few test failures that use hardcoded 'gcc' instead of
expected ${CC}. The change propagates detected ${CC} at
configure time to test scripts.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>