Do not set CMAKE_BUILD_TYPE if it is not defined
https://bugs.webkit.org/show_bug.cgi?id=114243

Reviewed by Geoffrey Garen.

Setting the CMAKE_BUILD_TYPE causes some problem with Visual Studio.
Adopt the current usage of the variable to better CMake style.

* CMakeLists.txt:
* Source/cmake/OptionsCommon.cmake:
* Source/cmake/OptionsEfl.cmake:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147992 268f45cc-cd09-0410-ab3c-d52691b4dbfc