Refactor CMake to use more modern paradigms (#741)

Remove 2.6-isms
Remove 2.8-isms
Bump CMake minimum version to 3.4

Disable some options when used as a subdirectory

Use `CONFIGURE_DEPENDS` with `file(GLOB)` when possible

Backport CMake 3.15's MSVC_RUNTIME_LIBRARY setting.
Set all compile options as generator expressions.
Set all find-package files to be installed to the correct file.

Remove `export(PACKAGE)`, as this has been deprecated.
Remove fat binary support
Remove manual setting of iPhone settings. These should be set by parent
projects.
Remove use of ExternalProject for a local use
Conditionally remove format target unless clang-format is found