[CMake] escape characters in strings

[Nico Schlömer]

Hi all,

in a CMake export file, I've got the line
```
SET(${LIBNAME}_CXX_FLAGS "${CMAKE_CXX_FLAGS}
${CMAKE_CXX_FLAGS_${CMAKE_BUILD_TYPE}}")
```
where the variables are replaced accordingly. A problem occurs if the
variable ${CMAKE_CXX_FLAGS_${CMAKE_BUILD_TYPE}} contains double
quotes; they should be escaped. I'm not sure if the quotation marks in
the above line could be left out.

What is the proper way of handling this?

Cheers,
Nico