make it more usable with MSYS and linux MinGW both (#48)

* make it more usable

* update README

Makefile

@@ -1,11 +1,14 @@
-CPREFIX = x86_64-w64-mingw32
+ifndef MSYSTEM
+	CPREFIX = x86_64-w64-mingw32-
+endif
+
 WINDIVERTHEADERS = ../../include
 WINDIVERTLIBS = ../binary
 
 TARGET = goodbyedpi.exe
 LIBS = -L$(WINDIVERTLIBS) -lWinDivert -lws2_32
-CC = $(CPREFIX)-gcc
-CCWINDRES = $(CPREFIX)-windres
+CC = $(CPREFIX)gcc
+CCWINDRES = $(CPREFIX)windres
 CFLAGS = -Wall -I$(WINDIVERTHEADERS) -L$(WINDIVERTLIBS) \
          -O2 -pie -fPIE -Wformat -Werror=format-security -D_FORTIFY_SOURCE=2
 LDFLAGS = -pie

README.md

@@ -65,11 +65,11 @@ This project can be build using **GNU Make** and [**mingw**](https://mingw-w64.o
 
 To build x86 exe run:
 
-`make CPREFIX=i686-w64-mingw32 WINDIVERTHEADERS=/path/to/windivert/include WINDIVERTLIBS=/path/to/windivert/x86`
+`make CPREFIX=i686-w64-mingw32- WINDIVERTHEADERS=/path/to/windivert/include WINDIVERTLIBS=/path/to/windivert/x86`
 
 And for x86_64:
 
-`make CPREFIX=x86_64-w64-mingw32 WINDIVERTHEADERS=/path/to/windivert/include WINDIVERTLIBS=/path/to/windivert/amd64`
+`make CPREFIX=x86_64-w64-mingw32- WINDIVERTHEADERS=/path/to/windivert/include WINDIVERTLIBS=/path/to/windivert/amd64`
 
 # How to install as Windows Service