Update build & install instructions for MSys2/Windows

.gitignore

@@ -1,2 +1,2 @@
 *.o
-git-crypt
+git-crypt*

INSTALL

@@ -63,10 +63,27 @@ Using the brew package manager, simply run:
 
 EXPERIMENTAL WINDOWS SUPPORT
 
-git-crypt should build on Windows with MinGW, although the build system
-is not yet finalized so you will need to pass your own CXX, CXXFLAGS, and
-LDFLAGS variables to make.  Additionally, Windows support is less tested
-and does not currently create key files with restrictive permissions,
-making it unsuitable for use on a multi-user system.  Windows support
-will mature in a future version of git-crypt.  Bug reports and patches
-are most welcome!
+git-crypt should  build on Windows with  MinGW.  Additionally, Windows
+support is  less tested and does  not currently create key  files with
+restrictive permissions, making it unsuitable  for use on a multi-user
+system.  Windows support will mature in a future version of git-crypt.
+Bug reports and patches are most welcome!
+
+1.  Install  a MinGW/MSys2 development environment,  for example using
+    https://github.com/orlp/dev-on-windows/wiki/Installing-GCC--&-MSYS2:
+	1.  Remove or rename `%USERPROFILE%\.bashrc` if you have one prior
+        to installation.
+	2.  Install  [MSys2](https://www.msys2.org/) & make sure  there is
+        no more update pending using `pacman -Syuu`.
+	3.  From  a MSYS2 MinGW  64-bit terminal, install  the development
+        environment:
+
+	$ pacman -S --needed base-devel mingw-w64-i686-toolchain mingw-w64-x86_64-toolchain \
+		git subversion mercurial \
+		mingw-w64-i686-cmake mingw-w64-x86_64-cmake
+
+2.  In a MSYS2 MinGW 64-bit terminal, run `make`
+3.  Manually  install the generated `git-crypt.exe`  somewhere in your
+    `%PATH%` or simply  run `make install` if you plan  to use it only
+    from MSys2
+

INSTALL.md

@@ -62,10 +62,26 @@ Using the brew package manager, simply run:
 
 ### Experimental Windows Support
 
-git-crypt should build on Windows with MinGW, although the build system
-is not yet finalized so you will need to pass your own CXX, CXXFLAGS, and
-LDFLAGS variables to make.  Additionally, Windows support is less tested
-and does not currently create key files with restrictive permissions,
-making it unsuitable for use on a multi-user system.  Windows support
-will mature in a future version of git-crypt.  Bug reports and patches
-are most welcome!
+git-crypt should  build on Windows with  MinGW.  Additionally, Windows
+support is  less tested and does  not currently create key  files with
+restrictive permissions, making it unsuitable  for use on a multi-user
+system.  Windows support will mature in a future version of git-crypt.
+Bug reports and patches are most welcome!
+
+1.  Install  a MinGW/MSys2 development environment,  for example using
+    https://github.com/orlp/dev-on-windows/wiki/Installing-GCC--&-MSYS2:
+	1.  Remove or rename `%USERPROFILE%\.bashrc` if you have one prior
+        to installation.
+	2.  Install  [MSys2](https://www.msys2.org/) & make sure  there is
+        no more update pending using `pacman -Syuu`.
+	3.  From  a MSYS2 MinGW  64-bit terminal, install  the development
+        environment:
+
+			$ pacman -S --needed base-devel mingw-w64-i686-toolchain mingw-w64-x86_64-toolchain \
+                    git subversion mercurial \
+                    mingw-w64-i686-cmake mingw-w64-x86_64-cmake
+
+2.  In a MSYS2 MinGW 64-bit terminal, run `make`
+3.  Manually  install the generated `git-crypt.exe`  somewhere in your
+    `%PATH%` or simply  run `make install` if you plan  to use it only
+    from MSys2

Makefile

@@ -10,9 +10,18 @@ PREFIX ?= /usr/local
 BINDIR ?= $(PREFIX)/bin
 MANDIR ?= $(PREFIX)/share/man
 
+ifeq ($(OS),Windows_NT)
+# MSys2 install instead of ./INSTALL
+INSTALL = /usr/bin/install
+else
+INSTALL = install
+endif
+
 ENABLE_MAN ?= no
 DOCBOOK_XSL ?= http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
 
+$(info OSTYPE=$(OSTYPE))
+
 OBJFILES = \
     git-crypt.o \
     commands.o \
@@ -82,12 +91,12 @@ INSTALL_TARGETS := install-bin $(INSTALL_MAN_TARGETS-$(ENABLE_MAN))
 install: $(INSTALL_TARGETS)
 
 install-bin: build-bin
-	install -d $(DESTDIR)$(BINDIR)
-	install -m 755 git-crypt $(DESTDIR)$(BINDIR)/
+	$(INSTALL) -d $(DESTDIR)$(BINDIR)
+	$(INSTALL) -m 755 git-crypt $(DESTDIR)$(BINDIR)/
 
 install-man: build-man
-	install -d $(DESTDIR)$(MANDIR)/man1
-	install -m 644 man/man1/git-crypt.1 $(DESTDIR)$(MANDIR)/man1/
+	$(INSTALL) -d $(DESTDIR)$(MANDIR)/man1
+	$(INSTALL) -m 644 man/man1/git-crypt.1 $(DESTDIR)$(MANDIR)/man1/
 
 .PHONY: all \
 	build build-bin build-man \