configure: add AC_LANG_SOURCE call within AC_COMPILE_IFELSE

Documentation states that AC_*_IFELSE has to use AC_LANG_SOURCE or
friends in order to generate the source code to compile.
AC_LINK_IFELSE already handles this, thus convert AC_COMPILE_IFELSE
to silence the final autoconf warnings.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>