Support to build both static and shared libraries

[dota17]

#593

[coveralls]

Coverage decreased (-0.2%) to 85.768% when pulling 70afc2b on dota17:static_shared into 8e3d3d5 on json-c:master.

[dota17]

Allow to build both static and shared libraries.

cmake -DBUILD_STATIC_LIBS=ON ..

[hawicz]

We should default to both static and shared being built, and describe -DBUILD_SHARED_LIBS=OFF and -DBUILD_STATIC_LIBS=OFF as simply turning the relevant build off, rather than having it say it turns the "other" libs build on.

[dota17]

ok

[hofnarrr]

Hi, there is some problems with the fix this branch provides. Not sure what's the "correct" way to submit fixes to other people's PR's, so I submitted a pull request to dota17's branch (dota17#90).

[dota17]

@hofnarrr Thanks a lot! I found the install error of share&static libraries before and was thinking how to fix it. Your fixs are very timely, ha-ha.