LONG_LONG_MAX is an obsolete GNU name for LLONG_MAX

Summary:
Apparently it originated from before C99 standardized to LLONG_*.
Nowadays everybody with a gcc > 3.0 should be fine, not only the
BSDs [1].
Also fixes a build error with musl libc.

[1] https://www.gnu.org/software/libc/manual/html_node/Range-of-Type.html#Range-of-Type

Test Plan:
Builds on Linux with glib
Builds on Linux with musl

Reviewers: mutlaqja

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D19401