fix: Fix build configure

[re2zero]

It miss the Qt MAJOR version if build with Qt5.

Log: Fix build configure.

[re2zero]

$ cmake -B build
-- Building KDSingleApplication 1.1.95 in Debug mode. Installing to /usr/local
-- The following REQUIRED packages have been found:

 * QT
 * Qt5Network
 * Qt5Gui (required version >= 5.11.3)
 * Qt5Widgets
 * Qt5
 * Qt5Core

-- Configuring done
CMake Error at src/CMakeLists.txt:19 (add_library):
  Target "kdsingleapplication" links to target "Qt::Core" but the target was
  not found.  Perhaps a find_package() call is missing for an IMPORTED
  target, or an ALIAS target is missing?


CMake Error at src/CMakeLists.txt:19 (add_library):
  Target "kdsingleapplication" links to target "Qt::Network" but the target
  was not found.  Perhaps a find_package() call is missing for an IMPORTED
  target, or an ALIAS target is missing?


CMake Error at examples/widgetsingleapplication/CMakeLists.txt:10 (add_executable):
  Target "widgetsingleapplication" links to target "Qt::Widgets" but the
  target was not found.  Perhaps a find_package() call is missing for an
  IMPORTED target, or an ALIAS target is missing?


CMake Error at examples/widgetsingleapplication/CMakeLists.txt:10 (add_executable):
  Target "widgetsingleapplication" links to target "Qt::Core" but the target
  was not found.  Perhaps a find_package() call is missing for an IMPORTED
  target, or an ALIAS target is missing?


-- Generating done
CMake Generate step failed.  Build files cannot be regenerated correctly.

$ qmake --version
QMake version 3.1
Using Qt version 5.11.3 in /usr/lib/x86_64-linux-gnu

Hello, I 'm very thanks your such kind solution. When I try to build it from sources, it reports above configure error when I try to build with Qt5 verison, add the Qt MAJOR should be common, is that correct? If it does not hve this issue on your side, please ignore this PR. Thanks.

[dantti]

I guess we might need to raise the minimum Qt version to ~5.15 due version less targets