[ISSUE] qmake from cross-compiled qt5 cannot find spec files

[ISSUE] qmake from cross-compiled qt5 cannot find spec files

[classabbyamp]

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New issue by classabbyamp on void-packages repository

https://github.com/void-linux/void-packages/issues/50731

Description:
### Is this a new report?

Yes

### System Info

Void 6.8.12_2 aarch64 Unknown notuptodate rDDFFFFFF

### Package(s) Affected

qt5-5.15.11+20231124_4, qt5-qmake-5.15.11+20231124_4

### Does a report exist for this bug with the project's home (upstream) and/or another distro?

no (packaging issue)

### Expected behaviour

with natively-built qt5:
```
$ /usr/lib/qt5/bin/qmake -query
QT_SYSROOT:
QT_INSTALL_PREFIX:/usr
QT_INSTALL_ARCHDATA:/usr/lib/qt5
QT_INSTALL_DATA:/usr/share/qt5
QT_INSTALL_DOCS:/usr/share/doc/qt5
QT_INSTALL_HEADERS:/usr/include/qt5
QT_INSTALL_LIBS:/usr/lib
QT_INSTALL_LIBEXECS:/usr/lib/qt5/libexec
QT_INSTALL_BINS:/usr/lib/qt5/bin
QT_INSTALL_TESTS:/usr/tests
QT_INSTALL_PLUGINS:/usr/lib/qt5/plugins
QT_INSTALL_IMPORTS:/usr/lib/qt5/imports
QT_INSTALL_QML:/usr/lib/qt5/qml
QT_INSTALL_TRANSLATIONS:/usr/share/qt5/translations
QT_INSTALL_CONFIGURATION:/etc/xdg
QT_INSTALL_EXAMPLES:/usr/share/qt5/examples
QT_INSTALL_DEMOS:/usr/share/qt5/examples
QT_HOST_PREFIX:/usr
QT_HOST_DATA:/usr/lib/qt5
QT_HOST_BINS:/usr/lib/qt5/bin
QT_HOST_LIBS:/usr/lib
QMAKE_SPEC:linux-g++
QMAKE_XSPEC:linux-g++
QMAKE_VERSION:3.1
QT_VERSION:5.15.11
```

```
$ qmake -recursive QGIS.pro
Reading /tmp/build/_core/_core.pro
```

### Actual behaviour

with cross-built qt5:
```
$ /usr/lib/qt5/bin/qmake -query
QT_SYSROOT:
QT_INSTALL_PREFIX:
QT_INSTALL_ARCHDATA:
QT_INSTALL_DATA:
QT_INSTALL_DOCS:
QT_INSTALL_HEADERS:
QT_INSTALL_LIBS:
QT_INSTALL_LIBEXECS:
QT_INSTALL_BINS:
QT_INSTALL_TESTS:
QT_INSTALL_PLUGINS:
QT_INSTALL_IMPORTS:
QT_INSTALL_QML:
QT_INSTALL_TRANSLATIONS:
QT_INSTALL_CONFIGURATION:
QT_INSTALL_EXAMPLES:
QT_INSTALL_DEMOS:
QT_HOST_PREFIX:
QT_HOST_DATA:
QT_HOST_BINS:
QT_HOST_LIBS:
QMAKE_SPEC:
QMAKE_XSPEC:
QMAKE_VERSION:3.1
QT_VERSION:5.15.11
```

```
$ qmake -recursive QGIS.pro
Could not find qmake spec ''.
Error processing project file: QGIS.pro
```

### Steps to reproduce

1. use cross-built qt5 to compile a program natively on aarch64 (like `qgis`)
2. observe failure when `sip-build` calls `qmake`