This unbreaks older GNU Make versions (up to 4.3), but leaves newer ones
(4.4 and up) broken.

This unfortunately includes Debian Unstable for now.

This is a regression in GNU Make and will need to be handled there,
since working around is nigh impossible (especially since values with
whitespace just can't be handled correctly).

It also means that OpenBSD's Make is essentially broken, but there's
nothing that can reasonably be done about this.

Quoting is obviously not eaten up by make and causing issues down the
line, so hope that values with spaces in them will be processed
correctly.

Especially things like LDFLAGS, QMAKE_OPTS and the like.

Fingers crossed.

Not passing values leads to builds breaking with newer make versions,
since they are now passing MAKEOVERRIDES as-is, leading to build
failures since instead of exporting variables, the data passed is now
interpreted as a make target instead.

Passing values should (hopefully?) be compatible with older make
versions, though, and we would have needed to do this for .MAKEFLAGS the
whole time anyway.

Only .MAKEOVERRIDES on FreeBSD and NetBSD is special in that this
variable actually expects a variable list, without values.

Makefile: support version switching through a QT_VERSION variable - might be overridden via environment variables or the make call.

The previous behavior was leading to compile failures on arm64 machines
(albeit only old distro versions, apparently), because QMAKE_LFLAGS is
being put on the command line *before* the actual object files to be
included/linked, which is an error for system libraries.

Builds will still fail after this commit, but that's okay.

Makefile: add comment explaining why we need to do this in the first place and why it's so complicated.

Makefile: the only way to prevent variables on the command line to be passed down seems to be to remove them from MAKEOVERRIDES.

.MAKEOVERRIDES is the FreeBSD-make-equivalent variable.

MAKEFLAGS has no effect on this behavior, so removing it again.

Makefile: reset to the original state and make sure that we don't pass CXXFLAGS on to the actual build process to not override the values generated by qmake.

Makefile: qmake seems to act weird if QMAKE_C(XX)FLAGS starts with a space followed by additional non-whitespace characters.

Refactor the variables a bit to work around this issue.

Makefile: add new QMAKE_OPTS variable so that packagers can pass additional options to qmake directly.

  - x2goclient.pro
  - Makefile
  - debian/rules
  - x2goclient.spec

  - COPYRIGHT.x2go-logos
  - Makefile
  - debian/x2goclient.install
  - macbuild.sh
  - x2goclient.pro
  - x2goclient.rc

  - resources.rcc
  - COPYRIGHT.x2go-logos
  - Makefile
  - debian/x2goclient.install
  - macbuild.sh
  - nsis/x2goclient.nsi
  - src/onmainwindow.cpp
  - src/appdialog.cpp
  - src/folderbutton.cpp
  - src/folderexplorer.cpp
  - src/sessionbutton.cpp
  - src/sessionexplorer.cpp
  - src/sessionmanagedialog.cpp
  - src/sessionwidget.cpp
  - src/ui/helpdialog.ui
  - src/xsettingswidget.cpp

Drop create_text.sh again, implement changelog copying in distro build files. Implement Git history creation for nightly builds in build scripts.

Make qmake-qt4 and lrelease path in Makefile easily replacable (as RHEL-5 does not have those tools in $PATH). + For EPEL-5 builds: replace full path to qmake-qt4 and lrelease. For EPEL-5 builds: replace full path to qmake-qt4 and lrelease.

Allow patching of qmake-qt4 executable path in Makefile. x2goclient.spec: Replace qmake-qt4 path in Makefile.

Allow QMAKE_* parameters that are needed for hardening x2goclient (see http://wiki.debian.org/Hardening).

A maxdepth of two will crush all temp files in the build directories.
Do not mess with other stuff, especially if users are building bundled
software inside a contrib/dist directory under the main tree.