tools: mkeficapsule: use pkg-config to get -luuid and -lgnutls

Instead of hardcoding -luuid -lgnutls as the flags needed to build
mkeficapsule, use pkg-config when available.

We gracefully fallback on the previous behavior of hardcoding -luuid
-lgnutls if pkg-config is not available or fails with an error.

Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>