[Bug 870455] cmake FindQt4 cannot find MinGW qconfig.pri

[Red Hat Bugzilla]

https://bugzilla.redhat.com/show_bug.cgi?id=870455

--- Comment #8 from Erik van Pienbroek  ---
The Fedora MinGW SIG recommends to use the mingw32-cmake / mingw64-cmake
wrappers to call CMake with the proper environment set. Internally these
wrappers call this RPM macro:

%mingw32_cmake %{mingw32_env} ; \
  if test -f CMakeLists.txt; then __mingw32_topdir=.; \\
  elif test -f ../CMakeLists.txt; then __mingw32_topdir=..; \\
  else __mingw32_topdir=""; fi; \\
  PATH=%{_prefix}/%{mingw32_target}/bin:$PATH %__cmake \\
        -DCMAKE_VERBOSE_MAKEFILE=ON \\
        -DCMAKE_INSTALL_PREFIX:PATH=%{mingw32_prefix} \\
        -DCMAKE_INSTALL_LIBDIR:PATH=%{mingw32_libdir} \\
        -DINCLUDE_INSTALL_DIR:PATH=%{mingw32_includedir} \\
        -DLIB_INSTALL_DIR:PATH=%{mingw32_libdir} \\
        -DSYSCONF_INSTALL_DIR:PATH=%{mingw32_sysconfdir} \\
        -DSHARE_INSTALL_PREFIX:PATH=%{mingw32_datadir} \\
        %{?_cmake_skip_rpath} \\
        -DBUILD_SHARED_LIBS:BOOL=ON \\
        -DCMAKE_TOOLCHAIN_FILE=/usr/share/mingw/Toolchain-mingw32.cmake \\
        ${MINGW_CMAKE_ARGS} \\
        ${MINGW32_CMAKE_ARGS} \\
        "$@" $__mingw32_topdir

If I understand the discussion above correctly then the QT_BINARY_DIR which is
set in Toolchain-mingw32.cmake / Toolchain-mingw64.cmake is redundant and
should be removed. I just tried to remove it from the toolchain file, but doing
so causes mingw32-cmake to not be able to detect to qt tools (like moc and rcc)
any more.

I think this is because QT_BINARY_DIR will be set to
/usr/i686-w64-mingw32/sys-root/mingw/bin internally by CMake. However, this
path only contains cross-compiled binaries. The native binaries belonging to
the cross-compiled Qt are in /usr/i686-w64-mingw32/bin (we don't want to mix
native and cross-compiled binaries in the same folder).

In the FindQt4.cmake file there's this snippet of code:

# ask qmake for the binary dir
  IF (NOT QT_BINARY_DIR  OR  QT_QMAKE_CHANGED)
    _qt4_query_qmake(QT_INSTALL_BINS qt_bins)
    SET(QT_BINARY_DIR ${qt_bins} CACHE INTERNAL "" FORCE)
  ENDIF (NOT QT_BINARY_DIR  OR  QT_QMAKE_CHANGED)

The qmake variable QT_INSTALL_BINS is used by Qt's build system to determine
where the (cross-)compiled binaries will be installed. I could try hack up the
mingw-qt package so that QT_INSTALL_BINS points to /usr/i686-w64-mingw32/bin
instead of /usr/i686-w64-mingw32/sys-root/mingw/bin but that isn't the most
elegant solution and I'm not sure what else will break because of this..

All qmake mkspecs profiles contain variables like QMAKE_MOC which points to the
full path where the moc binary is installed. Ideally CMake should be able to
use these variables instead of only searching for $QT_INSTALL_BINS/moc or
$PATH/moc. Unfortunately these value for these variables can't be extracted
with 'qmake-qt4 -query' so I guess a different solution should be searched..