protobuf: Update version to 2.6.1

- Unbroke MIPS support which got entirely lost since the upgrade from 2.4.1:
  now all ISAs should work (MIPS1 would need kernel emulation though, untested)
- Fixed host installation which was broken on all targets
- Updated source origin to github and related variables
- Kept mipseb patch exception and MIPS16:=0 exclusion (needed for BB and CC compilation)

Only tested on a ZyXEL NBG6716 router which is MIPS32 (MIPS74Kc), ar71xx target.

Signed-off-by: Guillaume Déflache <guillaume.deflache@ibwag.com>

libs/protobuf/patches/003-mips2andHigher-compile.patch

@@ -0,0 +1,11 @@
+--- -	2015-05-19 16:29:09.614344473 +0200
++++ protobuf-2.6.1/src/google/protobuf/stubs/atomicops_internals_mips_gcc.h	2015-05-19 13:49:52.127442746 +0200
+@@ -150,7 +150,7 @@
+ }
+ 
+ inline void MemoryBarrier() {
+-  __asm__ __volatile__("sync" : : : "memory");
++  __asm__ __volatile__(".set mips2; sync; .set mips0" : : : "memory");
+ }
+ 
+ inline void Acquire_Store(volatile Atomic32* ptr, Atomic32 value) {