Add option in CMAKE to not build documentation

[lal12]

I think it would be nice to add an option in CMake to not build the documentation. This can save a bit of time and more importantely it doesn't force you to install doxygen.

[dota17]

hi, I tried cmake .. many times with doxygen or non-installed doxygen. It looks like that forcing to install doxygen does not happen. In addition, cmake .. doesn't build the documentations and it just add the make target make doc. You could

cmake ..   
make
make doc  // generate the documentation

Maybe I should modify the message and make it clearer.

json-c/CMakeLists.txt

Line 353 in a063392

message("Doxygen need to be installed to generate the doxygen documentation")

[hawicz]

How about "Warning: doxygen not found, the 'doc' target will not be included"

[dota17]

good，I will submit this pr.

[dota17]

The message has been modified in #558，I will close this issue.