Fix CMake build on Solaris

When building LLVM/Clang on Solaris. The generated makefiles would
have an extraneous semi-colon character in them prior to this change
due to the way the 'CMAKE_CXX_FLAGS' variable was defined. Simply
adjusting the definition by moving the current CMAKE_CXX_FLAGS value
within the quotes solves the problem.

Patch by Art Haas!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@135361 91177308-0d34-0410-b5e6-96231b3b80d8