Don't hard-code include in include path

[jmairboeck]

If I have understood this correctly, this line

modern-cmake-sample/libjsonutils/CMakeLists.txt

Line 29 in 13671e8

$<INSTALL_INTERFACE:include>

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.