Also set QMAKE_EXECUTABLE.

While not strictly-cmake it will help to simplify qmake usage withing CMake
while crosbuilding.

debian/debtoolchainfilegen

@@ -51,4 +51,5 @@ set(CMAKE_C_COMPILER "$DEB_HOST_GNU_TYPE-gcc")
 set(CMAKE_CXX_COMPILER "$DEB_HOST_GNU_TYPE-g++")
 set(PKG_CONFIG_EXECUTABLE "$DEB_HOST_GNU_TYPE-pkg-config")
 set(PKGCONFIG_EXECUTABLE "$DEB_HOST_GNU_TYPE-pkg-config")
+set(QMAKE_EXECUTABLE "$DEB_HOST_GNU_TYPE-qmake")
 EOF