This fixes build with dev-build/slibtool

--- SDL2_gfx-1.0.4/configure.in
+++ SDL2_gfx-1.0.4/configure.in
@@ -58,6 +58,9 @@
 fi
 AC_CHECK_PROGS(WINDRES, [windres $hostaliaswindres $host_os-windres])
 
+AC_CHECK_LIBM
+AC_SUBST([LIBM])
+
 case "$host" in
     *-*-beos*)
         ac_default_prefix=/boot/develop/tools/gnupro
--- SDL2_gfx-1.0.4/Makefile.am
+++ SDL2_gfx-1.0.4/Makefile.am
@@ -24,6 +24,9 @@
 	-release $(LT_RELEASE)	\
 	-version-info $(LT_CURRENT):$(LT_REVISION):$(LT_AGE)
 
+libSDL2_gfx_la_LIBADD =		\
+	$(LIBM)
+
 %.o : %.rc
 	$(WINDRES) $< $@