libcap: fix static link

libcap: fix static link

With BR2_PREFER_STATIC_LIB, libcap fails to build:

  http://autobuild.buildroot.net/results/619bceb3491ecd2ed4e1ae552fdb237a0ed2fa47/build-end.log

This is due to the fact that it still tries to build a shared
library. This commit fixes that by adding a patch that makes libcap
provide install-shared/install-static targets. We also now only build
the $(@D)/libcap subdirectory, so removing the build of the progs
subdirectory is no longer needed.

[Peter: fixup description]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>