Since libwebp was switched to CMake, the .la files that told static
builds which libraries had to be linked with libwebp are no longer
generated, breaking static builds. By switching libtiff to CMake too,
we can use the CMake files instead. (This is necessary, but not
sufficient, for static builds to work — a further patch is required.)
This was tried once before (in 96d50327bc ("libtiff: export private
headers for freeimage")), and I reverted it (in
5df9305b60 ("libtiff: fix cross by switching back to autoconf")),
because at the time, it broke cross compilation of libtiff. Things
seem to have settled since then, and this now cross compiles
correctly.
Fixes: 909c491493 ("libwebp: build with CMake")
