package/rtptools: fix build for toolchains missinf SUN RPC headers

Use a configure check rather than checking explicitly for uClibc.

Make the patch a git patch at the same time.

Fixes:
    http://autobuild.buildroot.org/results/1cf/1cfe3e019a627e7c092dbd94e4b891179e35bedd/
    http://autobuild.buildroot.org/results/5ba/5bad40610316392c3f213c3e2a27ac76abd88de6/
    http://autobuild.buildroot.org/results/789/789ebdc6a96fc62ca5c98bdd6b90de20872ea221/


    ... and numerous similar build failures

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>