Compiler flag handlers (#6415)

This adds the ability for packages to apply compiler flags in one of
three ways: by injecting them into the compiler wrapper calls (the
default in this PR and previously the only automated choice);
exporting environment variable definitions for variables with
corresponding names (e.g. CPPFLAGS=...); providing them as arguments
to the build system (e.g. configure).

When applying compiler flags using build system arguments, a package
must implement the 'flags_to_build_system_args" function. This is
provided for CMake and autotools packages, so for packages which
subclass those build systems, they need only update their flag
handler method specify which compiler flags should be specified as
arguments to the build system.

Convenience methods are provided to specify that all flags be applied
in one of the 3 available ways, so a custom implementation is only
required if more than one method of applying compiler flags is
needed.

This also removes redundant build system definitions from tutorial
examples