Add -lm to 2015/hou/Makefile

Not all systems implicitly link in libm so this is necessary for those
systems.

2015/hou/Makefile

@@ -73,7 +73,7 @@ CFLAGS= ${CSTD} ${CWARN} ${ARCH} ${CDEFINE} ${CINCLUDE} ${OPT}
 
 # Libraries needed to build
 #
-LIBS=
+LIBS= -lm
 
 # C compiler to use
 #

thanks-for-fixes.md

@@ -1305,6 +1305,11 @@ that there are others as well but we don't seem to have them and he didn't
 provide them on his page. We've not yet contacted him for a copy but we will in
 time.
 
+## [2015/hou](2015/hou/prog.c) ([README.md](2015/hou/README.md))
+
+Cody added explicit linking of libm (`-lm`) for systems that do not do this
+(linux doesn't seem to but macOS does).
+
 
 ## [2015/yang](2015/yang/prog.c) ([README.md](2015/yang/README.md]))