Makefile: don't automatically set WINDOWSHELL unless PATH matches Windows norms

This is for MSYS2, which requires unix shell commands.

src/Makefile.d/detect.mk

@@ -29,7 +29,10 @@ $(call Print,$(_m))
 
 # go for a 32-bit sdl mingw exe by default
 MINGW:=1
+# cmd.exe uses native Windows semicolon delimited PATH
+ifneq (,$(findstring ;,$(PATH)))
 WINDOWSHELL:=1
+endif
 
 else # if you on the *nix