Don't hard-code `include` in include path

[jmairboeck]

If I have understood this correctly, this line

https://github.com/pabloariasal/modern-cmake-sample/blob/13671e862edda744a07ef423da9f45b3703b59be/libjsonutils/CMakeLists.txt#L29

should use ${CMAKE_INSTALL_INCLUDEDIR} instead of hard-coded include. There are systems which don't use include for that, for example Haiku which uses develop/headers instead.