Improve Makefile variables

Makefile

@@ -1,5 +1,6 @@
 exe = mmmaze
-CFLAGS = -std=c89 -pedantic -Wall -Wextra -D_XOPEN_SOURCE
+CPPFLAGS = -D_XOPEN_SOURCE
+CFLAGS = -std=c89 -pedantic -Wall -Wextra
 LDLIBS = -lcurses
 
 CC ?= cc
@@ -9,7 +10,7 @@ RM ?= rm -f
 all: $(exe)
 
 $(exe): src/*
-	$(CC) -o $@ $(CFLAGS) $(LDFLAGS) src/*.c $(LDLIBS)
+	$(CC) -o $@ $(CPPFLAGS) $(CFLAGS) $(LDFLAGS) src/*.c $(LDLIBS)
 
 .PHONY: clean
 clean:

README.md

@@ -34,7 +34,7 @@ It runs on the Windows Console specifically. My procedure to build it using
 MingW and PDCurses is as follows:
 
     make -C ../PDCurses/wincon -j CC=x86_64-w64-mingw32-gcc CFLAGS='-I.. -Os -flto' pdcurses.a &&
-    make exe=mmmaze.exe CC=x86_64-w64-mingw32-gcc CFLAGS='-std=c89 -pedantic -Wall -Wextra -I../PDCurses -Os -flto' LDFLAGS='-Wl,--gc-sections,--strip-all' LDLIBS='../PDCurses/wincon/pdcurses.a'
+    make exe=mmmaze.exe CC=x86_64-w64-mingw32-gcc CPPFLAGS='-I../PDCurses' CFLAGS='-std=c89 -pedantic -Wall -Wextra -Os -flto' LDFLAGS='-Wl,--gc-sections,--strip-all' LDLIBS='../PDCurses/wincon/pdcurses.a'
 
 (I keep the PDCurses root directory next to this project's root directory on my
 computer, which is why the above references `../PDCurses`.)