flac: unbreak powerpc e500 support

Add patch to avoid the enable/disable-debug options from unsetting -g
from CFLAGS in an incorrect way which breaks -mfloat-gprs=X (translit to
-mfloatprs=X).
It's possible to affect other less common CFLAGS with a '-g' in any
position.

Upstream status: sent different patch (configure.ac) - to avoid
autoreconfiguring.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>