Merge bitcoin#11711: bitcoin_qt.m4: Minor fixes and clean-ups.

06abcbf scripted-diff: Orthogonalize string quoting (Werner Lemberg)
e0496d3 bitcoin_qt.m4: Add missing dollar sign for variable. (Werner Lemberg)
079f4b2 bitcoin_qt.m4: Add missing braces around variables in autoconf messages. (Werner Lemberg)
8695315 bitcoin_qt.m4: Use correct M4 quoting characters. (Werner Lemberg)
db32a4f bitcoin_qt.m4: Improve QT_VERSION tests. (Werner Lemberg)

Pull request description:

  Replaces bitcoin#11222.

  Dropped the last commit, and converted e90d91c (now 06abcbf) into a scripted-diff using @theuni's suggestion.

Tree-SHA512: f2e1713bda96e8875be08839af914b24b3240f2eecf18cb268f83c82d965ebf544a0022af4f6f73b88b637a4fdd404a96b9fcf8e5bdd11c507b5bb425eeb7e1d