Add -lm to 2015/yang/Makefile

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

2015/yang/Makefile

@@ -74,7 +74,7 @@ CFLAGS= ${CSTD} ${CWARN} ${ARCH} ${CDEFINE} ${CINCLUDE} ${OPT}
 
 # Libraries needed to build
 #
-LIBS=
+LIBS= -lm
 
 # C compiler to use
 #

thanks-for-fixes.md

@@ -1311,6 +1311,8 @@ time.
 Cody fixed an unfortunate typo in the Makefile that was preventing some of the
 files from compiling properly, trying instead to compile already compiled code.
 
+He also added explicit linking of libm (`-lm`) for systems that do not do this
+(linux seems to not but macOS does).
 
 
 ## [2019/ciura](2019/ciura/prog.c) ([README.md](2019/ciura/README.md]))