Makefile.defs: add -lm to linker flags

- on some linux distros is not linked by default

src/Makefile.defs

@@ -1821,7 +1821,7 @@ ifeq ($(YACC),)
 endif
 YACC_FLAGS=-d -b core/cfg
 # on solaris add -lxnet (e.g. LIBS= -lxnet)
-LIBS= -ldl -lresolv
+LIBS= -ldl -lresolv -lm
 LIB_PREFIX:=lib
 LIB_SUFFIX:=.so