CMake build import: cmake variable provided on command line not honored

1. Configure the build:

   $ mkdir build
   $ cd build
   $ ~/usr/cmake-3.7.2-Linux-x86_64/bin/cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_INSTALL_PREFIX=../../installs/gcc54_rtti -DLLVM_ENABLE_RTTI=ON $LLVM_SOURCE_PATH
   

2. Build something

   $ make libclang-install
   

1. Open $LLVM_SOURCE_PATH/CMakeLists.txt in Qt Creator and import the build directory.

Check the cmake variables:

• CMAKE_BUILD_TYPE is now "Debug" instead of "RelWithDebInfo".
• CMAKE_INSTALL_PREFIX is now "/usr/local" instead of the provided one.
• LLVM_ENABLE_RTTI is OFF instead of ON.

Triggering a build from Qt Creator (same target as on command line: install-libclang) starts effectively a complete rebuild instead of a null build.