Remove unused *_ROOT globals from setup.py

[hugovk]

And simplify library root detection in build_extensions().

[radarhere]

In PIL, theses looked like

# --------------------------------------------------------------------
# Library pointers.
#
# Use None to look for the libraries in well-known library locations.
# Use a string to specify a single directory, for both the library and
# the include files.  Use a tuple to specify separate directories:
# (libpath, includepath).  Examples:
#
# JPEG_ROOT = "/home/libraries/jpeg-6b"
# TIFF_ROOT = "/opt/tiff/lib", "/opt/tiff/include"
#
# If you have "lib" and "include" directories under a common parent,
# you can use the "libinclude" helper:
#
# TIFF_ROOT = libinclude("/opt/tiff")

TCL_ROOT = None
JPEG_ROOT = None
ZLIB_ROOT = None
TIFF_ROOT = None
FREETYPE_ROOT = None
LCMS_ROOT = None

I think the expectation was for people building from source to patch these. https://github.com/conda-forge/pillow-feedstock/blob/main/recipe/patches/0003-add-XCB_ROOT.patch sort of does this.

You are asserting though that we have better mechanisms now, like ddb01ac, yes?