qmake property QT_SYSROOT treated as variable in qtwebengine/mkspecs/features/gn_generator.prf

Hello,

please see line 151 in qtwebengine/mkspecs/features/gn_generator.prf (source: qt-everywhere-src-5.10.0.tar.xz):

  !isEmpty(QT_SYSROOT) {

The QT_SYSROOT property is definitely non-empty in my cross-build setup, yet I noticed that this expression always evaluates to false and that the else-branch of this statement is processed.

Correct me if I'm wrong, but this looks like a syntactical problem because QT_SYSROOT is a qmake property and not a variable (a few lines down in gn_generator.prf it's also properly treated as a property, indicated by the use of the $$[QT_SYSROOT] instead of just $$QT_SYSROOT).

When adding a temporary variable assignment to this expression (this is semantically equal but with comprehensible access to the property's value) it yields true, as it's supposed to in my setup:

  QT_SYSROOT_VALUE = $$[QT_SYSROOT]
  !isEmpty(QT_SYSROOT_VALUE) {

I'm sure there are better ways to go about this, but this modification works as a demonstration as well as a proven fix for my cross-build (tested with Qt 5.9.3 and 5.10.0).

This specific code section of gn_generator.prf generates the values of the preprocessor variables QT_LIBDIR_EGL and QT_LIBDIR_GLES2 in <build>/qtwebengine/src/core/release/BUILD.gn (lines 439-440 in my case, use full-text search to locate them). Without the modification, the variable values are always prefixed with my host's QT_SYSROOT, otherwise not.

These two variable values are later used at run-time (meaning on my cross-build target) in qtwebengine/src/core/surface_factory_qt.cpp (lines 101 and 107) to locate the two shared libraries libEGL.so.1 and libGLESv2.so.2.

So without this modification, my host's QT_SYSROOT prefix ends up in the search path in libQt5WebEngineCore.so.5.10.0 when loading the two shared libraries at run-time on my cross-build target (which of course fails), while with it the QT_SYSROOT prefix is properly removed from that path.

Note that removing the QT_SYSROOT prefix is what gn_generator.prf is actually doing in the true-branch of the expression.