cmake: Use CMAKE_INSTALL_INCLUDEDIR indirection

[gjasny]

Like the header installation uses CMAKE_INSTALL_INCLUDEDIR:

curl/CMakeLists.txt

Lines 1516 to 1518 in 3831043

	install(DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/include/curl"
	DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
	FILES_MATCHING PATTERN "*.h")

the exported CMake configuration must use it as well to lookup the installed files in the same place.

[snikulov]

LGTM.

[bagder]

Thanks!