Add -fnested-functions to tests_CFLAGS

On some versions of GCC (e.g., Xcode's GCC on OS X), the -fnested-functions
flag needs to be passed as nested functions are disabled by default.

This fixes test compilation errors on OS X when using GCC 4.2.1.

Signed-off-by: Eddie Ringle <eddie@eringle.net>
Merge pull request #32 from eddieringle/add-nested-functions-cflag-to-tests

Add -fnested-functions to tests_CFLAGS