Previous versions of MinGW64 prepended an underscore to the symbol names, which was...

Previous versions of MinGW64 prepended an underscore to the symbol names, which was incompatible with the behavior of Visual C++ 64-bit.  MinGW64 1.0 changed its default behavior so that underscores are no longer prepended, so this patch removes the underscores from the SIMD symbol names when building with MinGW64, to prevent the build from breaking.  Those wishing to build TigerVNC with older MinGW64 versions will need to add -fno-leading-underscore to CFLAGS and CXXFLAGS.

git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/branches/1_1@4364 3789f03b-4d11-0410-bbf8-ca57d06f2519