fixes build issue for clang-3.2 and later (missing variable declaration ...

...in combination with -Werror causes build failure)

Clang 3.2 and newer complain if you define a variable in a header without an accompanying extern declaration. Since "cc" defaults to clang on 10-CURRENT, the build from master fails on my raspberry pi without this patch.

Note that this only causes a build failure due to the combination of -Wall and -Werror.