Update libmad pkgdef to use Git repo.

Fabian set up a repository with all the patches to get libmad running on
modern systems, so use that.

pkgdef/libmad.sh

@@ -1,25 +1,5 @@
 description "MPEG Audio Decoder library"
 check_library mad
-variant stable fetch_download \
+variant stable fetch_git https://github.com/chocolate-doom/libmad.git
-    ftp://ftp.mars.org/pub/mpeg/libmad-0.15.1b.tar.gz \
-    bbfac3ed6bfbc2823d3775ebb931087371e142bb0e9bb1bee51a76a6e0078690
 build_autotools
 
-prebuild_setup() {
-  cp configure configure.orig
-  # libmad hasn't had a new release in 14 years, so we must manually
-  # patch the configure script to remove all these compiler flags which
-  # don't work any more (especially in clang):
-  sed <configure.orig >configure '
-    s/-fforce-mem//
-    s/-fthread-jumps//
-    s/-fcse-follow-jumps//
-    s/-fcse-skip-blocks//
-    s/-fregmove//
-    s/-fexpensive-optimizations//
-    s/-fschedule-insns2//
-    s/-fstrength-reduce//
-    s/-march=i486//
-  '
-}
-