Look for headers in CMAKE_INSTALL_INCLUDEDIR rather than _LIBDIR

[BillyONeal]

Description

Put the correct include directory in the installed CMake configs.

The headers are installed to CMAKE_INSTALL_INCLUDEDIR here:

libsbp/c/src/CMakeLists.txt

Lines 81 to 83 in 0a93888

	INCLUDES DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
	)
	install(DIRECTORY ${PROJECT_SOURCE_DIR}/include/libsbp/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/libsbp)

but the target_include_directories uses CMAKE_INSTALL_LIBDIR.

This patch originally authored by @ethanmusser in microsoft/vcpkg#48274

@swift-nav/algint-team

API compatibility

Does this change introduce a API compatibility risk?

No.

API compatibility plan

If the above is "Yes", please detail the compatibility (or migration) plan:

N/A

JIRA Reference

N/A

[ethanmusser]

@BillyONeal One step ahead of ya! #1535

[BillyONeal]

@BillyONeal One step ahead of ya! #1535

Ah, I was looking for CMAKE_Xxx in the recent PRs so I missed it 😅. Closed in favor of yours, but added the location where the matching name comes from as a comment