[C/C++] CMake: Install interface is relative to the `lib` directory

[ethanmusser]

The sbp target's install interface is relative to the lib directory. Common convention places this relative to the include directory. As a result of the current configuration, library consumers will look in the lib directory for the headers.

libsbp/c/src/CMakeLists.txt

Line 56 in 0a93888

$<INSTALL_INTERFACE:${CMAKE_INSTALL_LIBDIR}>

Proposed Solution

Replace the above line with the following.

$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>