Makefile.w32: Remove Perl-regex grep dependency

msys grep does not support the -Po option, so use sed instead
README.md: better describe that building requires an msys/MinGW
environment

Makefile.w32

@@ -1,7 +1,8 @@
-GREP=grep
-VERSION:=$(shell $(GREP) -Po "(?<=\[)([0-9.]+.[0-9]+.[0-9]+)(?=\])" configure.ac)
+SED=sed
+VERSION:=$(shell "$(SED)" -n "s/[^[]*\[\([0-9]\+\.[0-9]\+\.[0-9]\+\)\],/\1/p" configure.ac)
 
 CC=gcc
+RM=/bin/rm
 
 SRCS = \
 	src/decompress.c \
@@ -23,11 +24,12 @@ TARGET = ag.exe
 
 all : $(TARGET)
 
-$(TARGET) : $(OBJS)
+# depend on configure.ac to account for version changes
+$(TARGET) : $(OBJS) configure.ac
 	$(CC) -o $@ $(OBJS) $(LIBS)
 
 .c.o :
 	$(CC) -c $(CFLAGS) -Isrc $< -o $@
 
 clean :
-	rm -f src/*.o $(TARGET)
+	$(RM) -f src/*.o $(TARGET)

README.md

@@ -115,9 +115,9 @@ or
 
         ./build.sh
 
-  On Windows:
+  On Windows (inside an msys/MinGW shell):
 
-        mingw32-make -f Makefile.w32
+        make -f Makefile.w32
 3. Make install:
 
         sudo make install