Fix static build issues and allow user to choose which library do they want

[makise-homura]

CMake does have a global BUILD_SHARED_LIBS variable which affects how add_library() works. E.g., when BUILD_SHARED_LIBS=ON (as it is usually on Windows, where you often link with shared Qt), there was no way to build clew statically, even if BUILD_SHARED_LIBRARY is set to OFF (which should lead to building a static library, according to documentation).

So, I did a fix that makes clew build like that:

• If either BUILD_SHARED_LIBRARY or BUILD_STATIC_LIBRARY is set to ON, this overrides global BUILD_SHARED_LIBS and allows clew to be built either shared or static, regardless of BUILD_SHARED_LIBS;
• If both are set to OFF (this is now the default), BUILD_SHARED_LIBS is used to determine which type of library does user want;
• Library can be built either static or shared, setting both BUILD_SHARED_LIBRARY and BUILD_STATIC_LIBRARY to ON is treated as error.

And also I removed this annoying "exported" message while building a shared library on WIN32, otherwise it really does not look like a warningless build.