fixed: using math functions in util, but failed to link libm.

The new utility functions added to libspnav 1.x use trigonometric and
sqrt functions from the math library, but linking to libm was previously
omitted from the makefile.

Closes bug report #20.

Makefile.in

@@ -11,7 +11,7 @@ libpaths = -L/usr/local/lib -L/usr/X11R6/lib
 CC ?= gcc
 AR ?= ar
 CFLAGS = $(opt) $(dbg) -std=c89 $(pic) -pedantic -Wall -fno-strict-aliasing $(incpaths) $(user_cflags)
-LDFLAGS = $(libpaths) $(user_ldflags) $(xlib)
+LDFLAGS = $(libpaths) $(user_ldflags) $(xlib) -lm
 
 ifeq ($(shell uname -s), Darwin)
 	lib_so = libspnav.dylib