package/mupdf: bump to version 1.18.0

package/mupdf: bump to version 1.18.0

It turns out the patch version of python-pymupdf doesn't have to match
mupdf's version (i.e. python-pymupdf 1.18.14 is compatible with mupdf
1.18.*) so remove the word "exactly" from the comment.

mupdf now has a new dependency: gumbo-parser.

gumbo-parser is included when downloading the sources for mupdf, but
instead we use the version provided by Buildroot.

While at it, take the chance to (based on previous review comments):

- Fix indentation in the hash file (two spaces).
- Switch to using the ".xz" archive.
- Add a conditional on freeglut.

Signed-off-by: Raphaël Mélotte <raphael.melotte@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>