Link libcompat with a library with math functions

We detect whether math functions are located in -lm in configure.ac.
Reuse the deliberable of this check and link libcompat with $(LIBM).
This is required at least for sincos() functions and it has been
reported as required on NetBSD 8.0.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

compat/Makefile.am

 pkglib_LTLIBRARIES = libcompat.la
 libcompat_la_SOURCES = dummy.c
-libcompat_la_LIBADD = $(LTLIBOBJS) $(LIBRT)
+libcompat_la_LIBADD = $(LTLIBOBJS) $(LIBRT) $(LIBM)
 libcompat_la_LDFLAGS = -no-undefined -static
 
 BUILT_SOURCES = dummy.c