bitcoin_qt.m4: Minor fixes and clean-ups.

Summary:

• bitcoin_qt.m4: Improve QT_VERSION tests.

Use '<QtCore/qconfig.h> and '<QtCore/qglobal.h>' for testing QT_VERSION.
This makes the tests work with both Qt4 and Qt5, even if '-fPIC' or '-fPIE'
is not used (the compiler might choke otherwise if QT_REDUCE_RELOCATIONS is
active).

• bitcoin_qt.m4: Use correct M4 quoting characters.
• bitcoin_qt.m4: Add missing braces around variables in autoconf messages.
• bitcoin_qt.m4: Add missing dollar sign for variable.
• scripted-diff: Orthogonalize string quoting

Add double qoutes to string tests where arguments could (theoretically)
contain spaces.

Remove double quotes where not necessary.

-BEGIN VERIFY SCRIPT-
sed -iE -e 's/"x\(yes\|no\|auto\)\?"/x\1/g' -e 's/test \(x\$[^ ]*\)/test "\1"/g' build-aux/m4/bitcoin_qt.m4
-END VERIFY SCRIPT-

This is a backport of Core PR11711

Depends on D1817

Test Plan:

../configure
make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1818