libmemcached: don't use -fPIE and -pie when linking statically

These compiler options shouldn't be used when linking statically, and in
particular FLAT binaries cannot use -fPIE and -pie code, so let's teach
libmemcached to not use such flags in those setups.

Fixes:

  http://autobuild.buildroot.org/results/8cb/8cbf57f9136cb42be31c88e0f1f32d3d2353e5ff/


  and many previous similar build failures

[Peter: rework/reword to do this based on BR2_STATIC_LIBS]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Tested-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>