Add -fno-common where supported (WAF only)

Add -fno-common where supported (WAF only)

Normally under UNIX, uninitialized non-static global variables get
placed in the "common" section, where they are merged at link time.
This means if two C files define "int debug", they will end up
referring to the same variable.  Or if one does "float level" and the
other does "int level" you'll get an accidental union.

Such bugs can be hard to track down; fortunately GCC offers
-fno-common to disable this feature.  It didn't reveal any places
which need fixing, however).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>