:farmer: Warnings in rendering windows 3 and 6

## Environment
* OS Version: Windows
* Source gz-rendering3 and gz-rendering6

## Description
CI builds show 20 warnings in the log output

## Steps to reproduce
1. Run a build in [gz-rendering3](https://build.osrfoundation.org/job/gz_rendering-ign-rendering3-win/) or [gz-rendering6](https://build.osrfoundation.org/job/gz_rendering-ign-rendering6-win/)
2. See 20 warnings appear

## Output

Reference build:
* https://build.osrfoundation.org/job/gz_rendering-ign-rendering3-win/36/consoleFull
* https://build.osrfoundation.org/job/gz_rendering-ign-rendering6-win/40/consoleFull

There isn't a direct link to the warnings. However, the log output shows:

```
-- Searching for swig - not found.
-- SKIP_PYBIND11 set - disabling python bindings
CMake Warning at C:/J/workspace/gz_rendering-ign-rendering3-win/ws/install/ignition-cmake2/share/cmake/ignition-cmake2/cmake2/IgnConfigureBuild.cmake:55 (message):
CONFIGURATION WARNINGS:
-- Swig is missing: Language interfaces are disabled.
Call Stack (most recent call first):
CMakeLists.txt:142 (ign_configure_build)
```

```
Checking Build System

Building Custom Rule C:/J/workspace/gz_rendering-ign-rendering3-win/ws/gz-math/src/CMakeLists.txt

cl : command line warning D9025: overriding '/W2' with '/w' [C:\J\workspace\gz_rendering-ign-rendering3-win\ws\build\ignition-math6\src\ignition-math6.vcxproj]
```
(and a lot more of these in different packages and files)

```
--- stderr: ignition-math6
CMake Warning at C:/J/workspace/gz_rendering-ign-rendering3-win/ws/install/ignition-cmake2/share/cmake/ignition-cmake2/cmake2/IgnConfigureBuild.cmake:55 (message):
CONFIGURATION WARNINGS:
-- Swig is missing: Language interfaces are disabled.
Call Stack (most recent call first):
CMakeLists.txt:142 (ign_configure_build)

---
Finished <<< ignition-math6 [46.4s]
```

```
--- stderr: ignition-rendering3
CMake Warning (dev) at C:/J/workspace/gz_rendering-ign-rendering3-win/ws/install/ignition-cmake2/share/cmake/ignition-cmake2/cmake2/IgnUtils.cmake:398 (message):
-- THIS MESSAGE IS INTENDED FOR IGNITION-RENDERING AUTHORS --
(IF YOU SEE THIS, PLEASE REPORT IT)

Could not find pkg-config information for IgnOGRE. It was not provided by
the find-module for the package, nor was it explicitly passed into the call
to ign_find_package(~). This is most likely an error in this project's use
of ign-cmake.
Call Stack (most recent call first):
CMakeLists.txt:72 (ign_find_package)
This warning is for project developers. Use -Wno-dev to suppress it.

CMake Warning (dev) at C:/J/workspace/gz_rendering-ign-rendering3-win/ws/install/ignition-cmake2/share/cmake/ignition-cmake2/cmake2/IgnUtils.cmake:398 (message):
-- THIS MESSAGE IS INTENDED FOR IGNITION-RENDERING AUTHORS --
(IF YOU SEE THIS, PLEASE REPORT IT)

Could not find pkg-config information for IgnOGRE2. It was not provided by
the find-module for the package, nor was it explicitly passed into the call
to ign_find_package(~). This is most likely an error in this project's use
of ign-cmake.
Call Stack (most recent call first):
CMakeLists.txt:100 (ign_find_package)
This warning is for project developers. Use -Wno-dev to suppress it.

CMake Warning at C:/J/workspace/gz_rendering-ign-rendering3-win/ws/install/ignition-cmake2/share/cmake/ignition-cmake2/cmake2/IgnConfigureBuild.cmake:55 (message):
CONFIGURATION WARNINGS:
-- Ogre 1.x versions greater than 1.9 are not officially supported.
-- The software might compile and even work but support from upstream
-- could be reduced to accepting patches for newer versions
Call Stack (most recent call first):
CMakeLists.txt:157 (ign_configure_build)

---
Finished <<< ignition-rendering3 [6min 35s]
```