subversion: ensure --disable-debug doesn't mangle CFLAGS

Fixes:
http://autobuild.buildroot.net/results/fc6/fc69a19c66462585449f7c4dad174d45a84e4947/
http://autobuild.buildroot.net/results/e04/e0471f2a9087d547840a7b18863289963e357b57/
http://autobuild.buildroot.net/results/bd8/bd8cdf976937c7b9029658871929f4be464b7a47/
http://autobuild.buildroot.net/results/652/652c3afe844e912061fbc5991e6fecad98ff6e6f/



And many more.

When --disable-debug is passed to configure, as is automatically done by the
autotools infrastructure since 822a7574 (infra: Move
--enable/--disable-debug to package/Makefile.in), the configure script will
try to strip debugging (-g) options from the C/CXXFLAGS. The logic to do so
is unfortunately buggy, so it ends up mangling options like
-mfloat-gprs=double that we use on certain PowerPC variants, breaking the
build.

Fix it by adjusting the sed regexp to be more selective in what it strips.
The package unfortunately doesn't cleanly autoreconf, so configure is
patched instead of configure.ac.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>