Add alt code to 1990/pjr

This alt code is recommended by the judges in the case the compiler
cannot compile main() due to lack of temporary stack space but it is
also recommended by the author as it does something else.

Format/typo check README.md file.

1990/pjr/Makefile

@@ -112,8 +112,8 @@ OBJ= ${PROG}.o
 DATA=
 TARGET= ${PROG}
 #
-ALT_OBJ=
-ALT_TARGET=
+ALT_OBJ= ${PROG}.alt.o
+ALT_TARGET= ${PROG}.alt
 
 
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