CMake: minimize the amount of include directories

When the OCE source directory has a long path, building with MSVC fails
because the Windows command length limit is exceded by some modules.

This fixes the issue by setting include directories per-target, and not
adding include directories of dependencies if OCE_COPY_HEADERS_BUILD is
enabled. (only available when using cmake 2.8.8 or greater)
Let TOOLKIT target depends on copy_${TOOLKIT}_headers

This fixes random build failures when OCE_USE_PCH=ON and
OCE_COPY_HEADERS_BUILD=ON.

When using precompiled headers with gcc, header files must be
copied before generating precompiled headers.
Merge pull request #512 from peterl94/command_limit_fix

CMake: minimize the amount of include directories