packaging qt applications

[David Craven]

Hi,

A package I'm working on depends on both qtbase and qtdeclarative. In
the configure phase I get an error that Qt5Config.cmake doesn't find
Qt5QmlConfig.cmake. When I use a snippet to remove this check I get:

```scheme
(snippet
   '(substitute* "CMakeLists.txt"
     (("find_package.*Qt5.*Qml Quick.*\n")
        "find_package(Qt5 ${REQUIRED_QT_VERSION} CONFIG REQUIRED Core
DBus Gui)\n")))))
```

```sh
CMake Error at 
/gnu/store/ngnz5l2v22a6wnr7lshlm02jykmhzl3z-qtdeclarative-5.6.1-1/lib/cmake/Qt5Qml/Qt5QmlConfig.cmake:87
(find_package):
  Could not find a package configuration file provided by "Qt5Network"
  (requested version 5.6.1) with any of the following names:

    Qt5NetworkConfig.cmake
    qt5network-config.cmake

  Add the installation prefix of "Qt5Network" to CMAKE_PREFIX_PATH or set
  "Qt5Network_DIR" to a directory containing one of the above files.  If
  "Qt5Network" provides a separate development package or SDK, be sure it has
  been installed.
Call Stack (most recent call first):
  declarative/compositor/CMakeLists.txt:2 (find_package)
```
The CMAKE_PREFIX_PATH seems to be set correctly and no matter what
CMAKE_PREFIX_PATH or other env variables I set I keep getting one or
the other error.

Has some else run into this issue and managed to fix it?

David