fix build failure on hurd-i386

libgroove fails to build on GNU/Hurd due to a name clash with OSX, both
are defining the __MACH__ keyword. This package has built previously,
and the latest building version is 4.1.1+dfsg-1+b1. This patch
fixes the build problems for version 4.2.0.

Signed-off-by: Andrew Kelley <superjoe30@gmail.com>