Add -lm to 2019/dogon/Makefile

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

2019/dogon/Makefile

@@ -76,7 +76,7 @@ CFLAGS= ${CSTD} ${CWARN} ${ARCH} ${CDEFINE} ${CINCLUDE} ${OPT}
 
 # Libraries needed to build
 #
-LIBS= -L ${X11_LIBDIR} -lX11
+LIBS= -L ${X11_LIBDIR} -lX11 -lm
 
 # C compiler to use
 #

thanks-for-fixes.md

@@ -1349,6 +1349,11 @@ Cody made the author's statement that the entry compiles cleanly true by fixing
 versions of C` (in main()). Not strictly necessary but if he's making fixes he
 might as well.
 
+## [2019/dogon](2019/dogon/prog.c) ([README.md](2019/dogon/README.md))
+
+Cody added explicit linking of libm (`-lm`) for systems that do not do this
+(linux does not seem to but macOS does).
+
 
 ## [2019/karns](2019/karns/prog.c) ([README.md](2019/karns/README.md]))