set qmake config via environment variables

i am trying to override QT_INSTALL_HEADERS with

qmake -set QT_INSTALL_HEADERS /custom/path/to/qt/include

but

qmake -query QT_INSTALL_HEADERS

is still returning the default path

the default path is set in qtbase/qmake/qmakelibraryinfo.cpp

result = QLibraryInfo::path(static_cast<QLibraryInfo::LibraryPath>(loc));

which is $prefix + "/include"

my patch allows me to override qmake variables by setting environment variables

export QT_INSTALL_HEADERS=/custom/path/to/qt/include
qmake -query QT_INSTALL_HEADERS

returns /custom/path/to/qt/include

setting a custom include path is needed in "exotic" linux distros like nixos linux or guix linux,
where qt libraries and qt headers can be in separate packages

for example

QT_INSTALL_PREFIX
/nix/store/36drd4c9gl0qs3mjjrx0c9klfx5nppv6-qtbase-6.2.1

QT_INSTALL_HEADERS
/nix/store/0jn9mafzpjgz3pza9fwvfxvgpfaj2y82-qtbase-6.2.1-dev/include

sample use of my patch in nixos
https://github.com/NixOS/nixpkgs/pull/141883/commits/fe75dce13582e3672606615ee63eb21ecf168dca

to fix the include path, i use

wrapProgram $dev/bin/qmake \
      --set QT_INSTALL_HEADERS $dev/include