* Fixed a bug causing config-specified compiler flags to be ignored.
Updated the compiler config so all flags are in a separate section.

* Updated the documentation for the `compilers.yaml` file spec.

* Implemented basic testing for the 'flags' section of compiler config.

* Fixed a few minor problems with the manual compiler config documentation.

* Don't clear LD_LIBRARY_PATH and friends from compiler wrappers

* remove debugging print

* Add new version property to handle joined version numbers

* Add unit test for new joined property

* Add documentation on version.up_to() and version.joined

* patch directive : fixed retrieval from urls fixes #1584

 - add support for 'gz' archives
 - fixed bugs with URL patches
 - updated nwchem

* patch directive : added checksum to UrlPatch

 - refactored classes in patch.py
 - updated nwchem

* patch directive : added caching

If Spec.dag_hash was called for the first time with a 'length'
specified, the cached hash was truncated. This ensures that the
full hash is cached.

* Removes the extra argument from Package.do_install while maintaining the changes in behavior pulled in #1603

* install : removed -i and -d shorthands (breaks backward compatibility)

* Change ':' to ','

* Add package bazel

* Add support for bazel-based builds

* Ignore GPG/PGP signatures/checksums when finding available downloads

* Remove duplicated basename

* Add platform-specific configuration scopes.

* Update `spack config` to use the new scope arguments.

* Major updates to Contribution Guide

* Grammar changes

* Fix missing/extra backticks

* Rewording, links, and tips added

* build_environment: allow compilers to set up an environment

* clang: mock up a toolchain directory for xcode

Some projects ignore CC and CXX flags and instead use xcode to find the
toolchain. Clang on Apple should set up the environment properly.
Arguably, every compiler could do this on Apple, but let's see how this
works out just for AppleClang for now.

The Documentation directory is ~1.7G and the excluded platforms add up
to about 7G. Ignoring swift saves another 500M. The resulting Xcode.app
copy is in the 2G range.

* compiler: set member variables early

This is required so that later methods can query things such as the
version of the compiler.

* compiler: support finding the real path of the compiler

On Apple, the /usr/bin compilers are actually wrapping tools themselves
which query xcrun for the currently selected Xcode installation. Pierce
this veil and get the real, full path the to underlying compilers
instead.

* icu4c: install with rpath

On macOS, icu installs with a library ID of the library name. Enabling
rpath makes its ID its full installed path which lets Qt5 link against
it successfully.

* qt: no -no-gtkstyle flag on Qt5 on macOS

* spack list : updated documentation

* spack list : removed space before ':'

This PR add documentation for the `suffixes` and `hash_length` options
for modules.yaml.

This resolves #1416.