Fix the MSYS2 build instructions; MSYS2_ARG_CONV_EXCL can no longer be empty.

Author: DavidEGrayson

README.md

@@ -62,7 +62,7 @@ After installing MSYS2, select "MinGW-w64 Win32 shell" from the Start Menu.  The
 
 Download the source code of this library and navigate to the top-level directory using `cd`.  Then run these commands to build the library and install it:
 
-    MSYS2_ARG_CONV_EXCL= cmake . -G"MSYS Makefiles" -DCMAKE_INSTALL_PREFIX=/mingw32
+    MSYS2_ARG_CONV_EXCL=- cmake . -G"MSYS Makefiles" -DCMAKE_INSTALL_PREFIX=/mingw32
     make install DESTDIR=/
 
 We currently do not provide any build files for Visual Studio.  You can use CMake to generate Visual Studio build files, and the library and its examples will probably compile, but we have not tested the resulting library.