New requirement: X11/Intrisic.h

[Vincent van Ravesteijn]

Since commit ([52fee355/lyxgit]; "Fix external middle-mouse pasting
with Qt5/X11"), I need X11/Intrinsic.h for which I had to install the
package libxt-devel.

Does this need a check in configure.ac ?, and/or a note in the RELEASE_NOTES ?

Vincent