Fix building with mingw

Specifically:
-Only use ms specific compiler flags when using msvc
-remove duplicate include dirs
-always define HAVE_CONFIG since it's generated from cmake
build fix for protobuf 2.5.0rc1
buildfix: windows compat since 22.5.0rc1
Merge pull request #1 from theor/master

Updated source list and README for protobuf-2.5.0rc1 unix and windows build.
Don't always build STATIC libraries

allow user to control with BUILD_SHARED_LIBS
Only add file to filelist if the file exists. Implement 'make install'

These conditional filelists help support multiple versions of the
library.
Add support for windows dll building.
Merge pull request #2 from exaurdon/master

Windows DLL Support
Don't force libprotobuf library to build as SHARED
Merge pull request #4 from wberrier/master

Get building with mingw on windows