Make use of upstream CMake infrastructure to detect the compiler toolchain

Summary:
Instead of having ad-hoc code for gcc, let CMake do its thing. It has a lot of
logic that we may be interested in, for example it will make the clang switch
much smoother.

Note it raises the minimum cmake to 3.7 for Android, which was released almost
2 years ago.

Test Plan: Built kalgebra on it using kdeorg/android-sdk

Reviewers: Frameworks, Build System, vkrause

Reviewed By: vkrause

Subscribers: vkrause

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D11776