Modern cmake & target for use with add_submodule

[Dawars]

Based on #15

The main consideration was to be able to be added as a submodule to other projects and link by target:

add_subdirectory(libs/libraw)
target_link_libraries(ProjectName PRIVATE libraw::libraw)

The minimum supported cmake version is 3.12 because of FIND_PACKAGE(JPEG) and because it's easy to install.

A small change is that the repo now contains libraw as a submodule and it doesn't have to be manually downloaded. This is in order to ease the usage by tools that expect the repo to work out of the box.

[letmaik]

Looks good now. Thanks for working through the changes! I'll hold off merging the PR until I validated this in rawpy's CI.

[letmaik]

I checked rawpy's CI and had to do the following changes:

• Update CMake to >= 3.12
  • for Windows I still had 3.7
  • for CentOS 6 (manylinux docker image), the package manager version is 2.8, I now install a newer version from cmake.org
• Add -DENABLE_OPENMP=OFF for macOS builds
  • This is because this PR makes it a failure if OpenMP is not found, and the default is ENABLE_OPENMP=ON

The CMake update is fine. The OpenMP update is a minor semantic break but I think this is good as otherwise there is no way to verify that you're actually getting OpenMP if you request it.

I'm going to merge this now. Thanks again for working on this, very much appreciated and long overdue :)