diags: remove overly aggressive copy+paste

diags/*/Makefile copied from gpio-mailbox/Makefile.
Remove uneeded portions of the Makefiles.

In diags/common, remove the compilation of both *.c and ../common/*.c,
as that ends compiling the same files twice. This should fix flakey
parallel builds.

Add -Werror, to keep us honest. Remove -std=c99, as that cannot
compile struct ifreq without errors:
http://stackoverflow.com/questions/10433982/why-does-c99-complain-about-storage-sizes

Change-Id: I9df9e2496b8552c066070a603a6f7469d488c1ca
4 files changed