Never allow -w.

Apparently -w will disable all warnings on GCC regardless of ordering
(clang will still respect ordering so warnings that are enabled after
-w are still respected). This is insane. Strip -w from the cflags.
Anyone that wants this flag should be turning off the specific
warnings (or just fix them), not disabling all warnings.

Change-Id: I2ba065637dfdc192921da4d9adbdc63b728c166f