Add documentation on build systems (#5015)

Spack provides a number of classes based on commonly-used build systems
that users can extend when writing packages; the classes provide functionality
to perform the actions relevant to the build system (e.g. running "configure" for
an Autotools-based package). This adds documentation for classes supporting the
following build systems:

* Makefile
* Autotools
* CMake
* QMake
* SCons
* Waf

This includes build systems for managing extensions of the following packages:

* Perl
* Python
* R
* Octave

This also adds documentation on implementing packages that use a custom build
system (e.g. Perl/CMake).

Spack also provides extendable classes which aggregate functionality for related
sets of packages, e.g. those using CUDA. Documentation is added for
CudaPackage.