handle MSVC flag warnings

[jmckenna]

• increases cmake_minimum_required to version 3.15, as the related MSVC flag policy was added in 3.15
  • see https://cmake.org/cmake/help/latest/policy/CMP0092.html
• this solves 707 repeated warnings with MSVC of Command line warning D9025 : overriding '/W3' with '/W4' such as:

[ 16%] Building CXX object CMakeFiles/geos.dir/src/geomgraph/Depth.cpp.obj
cl : Command line warning D9025 : overriding '/W3' with '/W4'
Depth.cpp
[ 16%] Building CXX object CMakeFiles/geos.dir/src/geomgraph/DirectedEdge.cpp.obj
cl : Command line warning D9025 : overriding '/W3' with '/W4'
DirectedEdge.cpp
[ 16%] Building CXX object CMakeFiles/geos.dir/src/geomgraph/DirectedEdgeStar.cpp.obj
cl : Command line warning D9025 : overriding '/W3' with '/W4'
DirectedEdgeStar.cpp
[ 16%] Building CXX object CMakeFiles/geos.dir/src/geomgraph/Edge.cpp.obj
cl : Command line warning D9025 : overriding '/W3' with '/W4'
Edge.cpp
[ 16%] Building CXX object CMakeFiles/geos.dir/src/geomgraph/EdgeEnd.cpp.obj
cl : Command line warning D9025 : overriding '/W3' with '/W4'
EdgeEnd.cpp

[robe2]

@jmckenna What version of VS are you running? or what switches are you using? Not seeing any notices or errors in our VS 2022 and VS 2019 bots so
I'm hesitant to upgrade CMake just for this, because then I think Ubuntu 20.04 and Debian 10 will no longer be able to build. Granted those are old systems and probably hopefully won't be used much by the time we release GEOS 3.13 but I think still relatively widely used.

[jmckenna]

@robe no problem at all, closing.

[jmckenna]

Coming back here with VS 2022, great that the easy fix is discoverable now through this ticket, and sometimes that is just as good for packagers (avoids a firestorm debate ha). Have a nice weekend all.

[jmckenna]

Great to see that this ticket is still discoverable and still used by other packagers. Sometimes this is just as good as a merged pull request (see recent thread)