contrib/mpg123: link with libm as appropriate (refs #16385)

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

contrib/src/mpg123/mpg123-libm.patch

+--- trunk/configure.ac	2015/09/06 15:59:04	3820
++++ trunk/configure.ac	2015/09/07 07:59:55	3827
+@@ -93,6 +93,8 @@
+ AM_PROG_CC_C_O
+ AC_PROG_CPP
+ AC_PROG_INSTALL
++AC_CHECK_LIBM
++AC_SUBST(LIBM)
+ dnl "Checking for egrep is broken after removal of libltdl stuff... checks use $EGREP, so searching it here."
+ AC_PROG_EGREP
+ AC_C_CONST
+--- mpg123/libmpg123.pc.in.orig	2015-08-12 01:33:47.000000000 -0400
++++ mpg123/libmpg123.pc.in	2016-02-03 20:40:25.335615001 -0500
+@@ -7,5 +7,5 @@
+ Description: An optimised MPEG Audio decoder
+ Requires: 
+ Version: @PACKAGE_VERSION@
+-Libs: -L${libdir} -lmpg123 
++Libs: -L${libdir} -lmpg123 @LIBM@
+ Cflags: -I${includedir} 

contrib/src/mpg123/rules.mak

@@ -23,6 +23,7 @@ $(TARBALLS)/mpg123-$(MPG123_VERSION).tar.bz2:
 mpg123: mpg123-$(MPG123_VERSION).tar.bz2 .sum-mpg123
 	$(UNPACK)
 	$(APPLY) $(SRC)/mpg123/no-programs.patch
+	$(APPLY) $(SRC)/mpg123/mpg123-libm.patch
 ifdef HAVE_ANDROID
 	$(APPLY) $(SRC)/mpg123/mpg123_android_off_t.patch
 endif