doxygen.cmake : make tagfiles processing more consistent

[ManifoldFR]

• process list as a comma-separated native CMake list (less awkward)
• fix extraneous whitespaces
• always use absolute paths (allows dedupe to work)
• remove custom REMOVE_DUPLICATES() function

[ManifoldFR]

As discussed with @jorisv, the following lines:

jrl-cmakemodules/doxygen.cmake

Lines 642 to 647 in 4d66e41

	if(EXISTS ${PROJECT_BINARY_DIR}/doc/${PROJECT_NAME}.doxytag)
	install(
	FILES ${PROJECT_BINARY_DIR}/doc/${PROJECT_NAME}.doxytag
	DESTINATION ${CMAKE_INSTALL_FULL_DOCDIR}/doxygen-html
	)
	endif()

Mess up the installation of doxytags when doing cmake <insert-cmake-args-here> && make install.
Because on the first CMake configuration, the doxytag file in build/doc doesn't exist yet, so the install() for it is not registered.

This change was made in 9fead16

[ManifoldFR]

We've found a workaround with @jorisv, which is to make the doxytag file's build-tree relative location the same as its install-tree location -- that is, doc/doxygen-html. Before, the locations would be <build-dir>/doc/${PROJECT_NAME}.doxytag and <install-dir>/doxygen-html/${PROJECT_NAME}.doxytag.

We gain:

• better consistency
• no need that the tagfile exists before installing (let it be carried with the doxygen-html directory which is installed wholesale)
• easier to clean up