Minimum cmake version is too low

[godbyk]

The minimum cmake version required is currently set to 2.8.5. However, the target_include_directories command (found in src/lib_json/CMakeLists.txt lines 53 and 60) requires at least cmake version 2.8.11.

The first occurrence (line 53) was recently worked around by a pull request (commit 8dd32e1), but the second occurrence still causes problems for folks using cmake versions between 2.8.5 and 2.8.10.

Options to resolve this are to either increase the minimum cmake version required (see CMakeLists.txt, line 1) or add a workaround to line 60 in src/lib_json/CMakeLists.txt.