Makefile.defs: add -lm to linker flags

- on some linux distros is not linked by default

(cherry picked from commit f12492c231e1f3a08994727e3e8263886f1f2fbb)

src/Makefile.defs

@@ -1775,7 +1775,7 @@ 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