Clang has its own <tgmath.h> and doesn't use <tgmath.h> from glibc. Pass
"-I." to compiler only if $($(<F)-no-include-dot) are undefined. Define
it to yes for tgmath tests when testing with Clang.
Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Sam James <sam@gentoo.org>