cmake: do not quote variables for config.h anymore

v2: incorporate Roland's nitpicks (I hope)
v3: It was decided that it's better to change the default behavior
v4: make sure the CMake syntax is correct even if the value of the
    variable contains spaces (if it contains double quotes it's a
    different matter, but that's pretty much a corner case, IMHO)
v5: properly escape backslash and double quote characters in the cmake
    syntax
Merge pull request #329 from andlaus/unquoted_config_vars

cmake: support defining unquoted variables for config.h