Add incremental build support
Add incremental tar extraction
Add incremental configuration
Remove cross-newlib as a install-tools dep

cross-newlib is a cross-gcc dep so it will still be run. Pulling
cross-newlib up to a install-tools dep causes gross-gcc to recompile.
Add --enable-static configure flag for mpfr and mpc
Set install-deps as phony
Ignore libs directory
Add -Wno-error=return-type CFLAG for building gdb with clang
Merge pull request #3 from pebble/feature-incremental-build

Feature incremental build