support Ninja cmake generator on msys2

Add a hack to the root CMakeLists.txt to SET(MSYS ON) for the Ninja
generator when running under msys2, otherwise it cannot find libs.

The quoting fixes from #70 allow all of this to work.

CMakeLists.txt

@@ -109,6 +109,11 @@ ELSE()
     ADD_DEFINITIONS(-DNDEBUG)
 ENDIF()
 
+# hack for ninja in msys2
+IF(WIN32 AND CMAKE_GENERATOR STREQUAL Ninja AND (NOT $ENV{MSYSTEM} STREQUAL ""))
+    SET(MSYS ON)
+ENDIF()
+
 # Add support for Homebrew, MacPorts and Fink on OS X
 IF(APPLE)
     INCLUDE(MacPackageManagers)

README.md

@@ -24,6 +24,8 @@ make -j10
 `./installdeps` is supported on MSys2, Linux (Debian/Ubuntu, Fedora or Arch
 Linux) and Mac OS X (homebrew, macports or fink.)
 
+The Ninja cmake generator is also now supported, including on msys2.
+
 If your OS is not supported, you will need the following:
 
 - c++ compiler and binutils