cmake: bump minimum cmake version to 3.0 + add target_include_directories

[madebr]

1. Bumping CMake minimum version to 3.0 gets rid of these warnings:

   CMake Deprecation Warning at CMakeLists.txt:12 (cmake_minimum_required):
     Compatibility with CMake < 2.8.12 will be removed from a future version of
     CMake.
   
     Update the VERSION argument <min> value or use a ...<max> suffix to tell
     CMake that the project does not need compatibility with older versions.
   

   and

   CMake Warning (dev) at CMakeLists.txt:15 (project):
     Policy CMP0048 is not set: project() command manages VERSION variables.
     Run "cmake --help-policy CMP0048" for policy details.  Use the cmake_policy
     command to set the policy and suppress this warning.
   
     The following variable(s) would be set to empty:
   
       CMAKE_PROJECT_VERSION
       CMAKE_PROJECT_VERSION_MAJOR
       CMAKE_PROJECT_VERSION_MINOR
       CMAKE_PROJECT_VERSION_PATCH
   This warning is for project developers.  Use -Wno-dev to suppress it.
   

2. use list(APPEND) where applicable.
3. use WIN32.
   Also, the comment "we're about to move to Win64" is outdated.
4. conditionally enable c++
5. Create PhysFS::PhysFS and PhysFS::PhysFS-static targets, and export those via the installed cmake packages.
6. Use target_include_directories(tgt PUBLIC "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}")
   This fixes CMake: Use target_include_directories() #14
   (You need to use a generator expression because you don't want to make build machine paths to appear in the exported cmake package)

Fixes #14

[icculus]

I confess that CMake scares the crap out of me, so I'm taking it on faith that the "PhysFS::" namespace stuff isn't going to cause problems...minor questions and notes aside, most of this PR looks pretty good.

[madebr]

I've added MinGW to the ci matrix (using a pre-installed toolchain on the windows image)