Configure should return an error with not supported versions of QT libraries #15688

While configuring with a non-supported version of QT libraries, configure should exit with an error.

Environment:

$ git log --oneline -1
9e7dc68 Merge [#15616](/bitcoin-bitcoin/15616/): rpc: Clarify decodescript RPCResult doc
$ ls /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5.3.2
/usr/lib/x86_64-linux-gnu/libQt5Gui.so.5.3.2
$ /usr/lib/x86_64-linux-gnu/qt5/bin/moc --version
moc 5.3.2
$ uname -srv
Linux 4.9.0-0.bpo.7-amd64 [#1](/bitcoin-bitcoin/1/) SMP Debian 4.9.110-3+deb9u2~deb8u1 (2018-08-14)

Steps to replicate:

$ ./autogen.sh
$ ./configure --with-incompatible-bdb >/dev/null 2>/dev/null && echo OK
OK

Expected behavior: configure should exit with an error due to version of QT libraries not supported.

Details:

$ ./configure --with-incompatible-bdb | grep -i qt
configure: WARNING: Found Berkeley DB other than 4.8; wallets opened by this build will not be portable!
checking for QT5... yes
checking for QT_TEST... yes
checking for QT_DBUS... yes
checking for static Qt... no
checking whether -fPIE can be used with this Qt config... no
checking for moc-qt5... no
checking for moc... /usr/lib/x86_64-linux-gnu/qt5/bin/moc
checking for uic-qt5... no
checking for uic... /usr/lib/x86_64-linux-gnu/qt5/bin/uic
checking for rcc-qt5... no
checking for rcc... /usr/lib/x86_64-linux-gnu/qt5/bin/rcc
checking for lrelease-qt5... no
checking for lrelease... /usr/lib/x86_64-linux-gnu/qt5/bin/lrelease
checking for lupdate-qt5... no
checking for lupdate... /usr/lib/x86_64-linux-gnu/qt5/bin/lupdate
checking whether to build Bitcoin Core GUI... yes (Qt5)
configure: WARNING: libzmq version 4.x or greater not found, disabling
checking whether to build test_bitcoin-qt... yes
config.status: creating share/qt/Info.plist
  with gui / qt = yes

There is a check, but for some reason it doesn't work

https://github.com/bitcoin/bitcoin/blob/fd46c4c0018c41d36cd892ccb47485b572d65837/build-aux/m4/bitcoin_qt.m4#L249-L267

Note to self: check checks before making a PR...

Yes @MarcoFalke , I've seen the check, but somehow that part of the code is never used (it is used only when there is no pkg-config, and this is forbidden on linux systems). On top of that, in the code there is no evaluation of the QT libraries version when pkg-config is active (again, pkg-config is always active on linux).

I tested it with two systems (Debian Jessie and Debian Stretch), with depends and without, with pkgconfig and without (by embedding in the code not to use pkgconfig, like we do with $host==*mingw*) and adding --with-gui=qt5 or not. Here are the results:

Results:

• Yes (1):

checking for QT5... yes
checking for QT_TEST... yes
checking for QT_DBUS... yes
checking for static Qt... no
checking whether -fPIE can be used with this Qt config... no
checking for moc-qt5... no
checking for moc... /usr/lib/x86_64-linux-gnu/qt5/bin/moc
checking for uic-qt5... no
checking for uic... /usr/lib/x86_64-linux-gnu/qt5/bin/uic
checking for rcc-qt5... no
checking for rcc... /usr/lib/x86_64-linux-gnu/qt5/bin/rcc
checking for lrelease-qt5... no
checking for lrelease... /usr/lib/x86_64-linux-gnu/qt5/bin/lrelease
checking for lupdate-qt5... no
checking for lupdate... /usr/lib/x86_64-linux-gnu/qt5/bin/lupdate
checking whether to build Bitcoin Core GUI... yes (Qt5)
checking whether to build test_bitcoin-qt... yes
config.status: creating share/qt/Info.plist
with gui / qt = yes

• No (2): This happens every time there is pkg-config=no (hardcoded by me) and there is no option --with-gui=qt5.

checking QtPlugin usability... no
configure: WARNING: QtCore headers missing; bitcoin-qt frontend will not be built
checking QtPlugin presence... no
checking for QtPlugin... no
checking whether to build Bitcoin Core GUI... no (Qt5)
with gui / qt = no

• Yes (3): This happens every time pkg-config is activated (it's always pre-activated on linux), and depends is on and there is the option --with-gui=qt5.

checking for QT5... yes
checking for QT_TEST... yes
checking for QT_DBUS... yes
checking for static Qt... yes
checking for QTFONTDATABASE... yes
checking for QTEVENTDISPATCHER... yes
checking for QTTHEME... yes
checking for QTDEVICEDISCOVERY... yes
checking for QTACCESSIBILITY... yes
checking for QTFB... yes
checking for QTXCBQPA... yes
checking for static Qt plugins: -lqminimal... yes
checking for static Qt plugins: -lqxcb -lxcb-static... yes
checking whether -fPIE can be used with this Qt config... yes
checking for moc-qt5... no
checking for uic-qt5... no
checking for rcc-qt5... no
checking for lrelease-qt5... no
checking for lupdate-qt5... no
checking whether to build Bitcoin Core GUI... yes (Qt5)
checking whether to build test_bitcoin-qt... yes
with gui / qt = yes

• Error (4): This happens every time I use the option --with-gui=qt5 with pkg-config not active (it is always active for linux systems):

checking QtPlugin usability... no
checking QtPlugin presence... no
checking for QtPlugin... no
configure: error: QtCore headers missing

I will add a PR, that fixes the issue, in a few hours. I'm testing it with different configurations.

Removing "GUI" and "good first issue" here, as this is a build system issue, and there's a PR open that will likely take care of this.