GNU-like handling of installation prefix et al.

Details:
- Changed the default installation prefix from $HOME/lib to /usr/local.
- Modified the way configure internally handles the prefix, libdir,
  includedir, and sharedir (and also added an --exec-prefix option).
  The defaults to these variables are set as follows:
    prefix:      /usr/local
    exec_prefix: ${prefix}
    libdir:      ${exec_prefix}/lib
    includedir:  ${prefix}/include
    sharedir:    ${prefix}/share
  The key change, aside from the addition of exec_prefix and its use to
  define the default to libdir, is that the variables are substituted
  into config.mk with quoting that delays evaluation, meaning the
  substituted values may contain unevaluated references to other
  variables (namely, ${prefix} and ${exec_prefix}). This more closely
  follows GNU conventions, including those used by GNU autoconf, and
  also allows make to override any one of the variables *after*
  configure has already been run (e.g. during 'make install').
- Updates to build/config.mk.in pursuant to above changes.
- Updates to output of 'configure --help' pursuant to above changes.
- Updated docs/BuildSystem.md to reflect the new default installation
  prefix, as well as mention EXECPREFIX and SHAREDIR.
- Changed the definitions of the UNINSTALL_OLD_* variables in the
  top-level Makefile to use $(wildcard ...) instead of 'find'. This
  was motivated by the new way of handling prefix and friends, which
  leads to the 'find' command being run on /usr/local (by default),
  which can take a while almost never yielding any benefit (since the
  user will very rarely use the uninstall-old targets).
- Removed periods from the end of descriptive output statements (i.e.,
  non-verbose output) since those statements often end with file or
  directory paths, which get confusing to read when puctuated by a
  period.
- Trival change to 'make showconfig' output.
- Removed my name from 'configure --help'. (Many have contributed to it
  over the years.)
- In configure script, changed the default state of threading_model
  variable from 'no' to 'off' to match that of debug_type, where there
  are similarly more than two valid states. ('no' is still accepted
  if given via the --enable-debug= option, though it will be
  standardized to 'off' prior to config.mk being written out.)
- Minor variable name change in flatten-headers.py that was intended for
  32812ff.
- CREDITS file update.

docs/BuildSystem.md

@@ -90,7 +90,7 @@ However, as of this writing, only a limited number of architectures are detected
 
 Upon running configure, you will get output similar to the following. The exact output will depend on whether you cloned BLIS from a `git` repository or whether you obtained BLIS via a downloadable tarball from the [releases](https://github.com/flame/blis/releases) page.
 ```
-$ ./configure haswell
+$ ./configure --prefix=$HOME/blis haswell
 configure: using 'gcc' compiler.
 configure: found gcc version 5.4.0 (maj: 5, min: 4, rev: 0).
 configure: checking for blacklisted configurations due to gcc 5.4.0.
@@ -169,17 +169,11 @@ The installation prefix can be specified via the `--prefix=PREFIX` option:
 ```
 $ ./configure --prefix=/usr <configname>
 ```
-This will cause libraries to eventually be installed (via `make install`) to `PREFIX/lib` and development headers to be installed to `PREFIX/include`. (The default value of `PREFIX` is `$(HOME)/blis`.) You can also specify the library install directory separately from the development header install directory with the `--libdir=LIBDIR` and `--includedir=INCDIR` options, respectively:
+This will cause libraries to eventually be installed (via `make install`) to `PREFIX/lib` and development headers to be installed to `PREFIX/include`. (The default value of `PREFIX` is `/usr/local`.) You can also specify the library install directory separately from the development header install directory with the `--libdir=LIBDIR` and `--includedir=INCDIR` options, respectively:
 ```
 $ ./configure --libdir=/usr/lib --includedir=/usr/include <configname>
 ```
-The `--libdir=LIBDIR` and `--includedir=INCDIR` options will override any `PREFIX` path, whether it was specified explicitly via `--prefix` or implicitly (via the default). That is, `LIBDIR` defaults to `PREFIX/lib` and `INCDIR` defaults to `PREFIX/include`, but each will be overriden by their respective `--libdir`/`--includedir` options. So,
-```
-$ ./configure --libdir=/usr/lib <configname>
-
-```
-will configure BLIS to install libraries to `/usr/lib` and header files to the default location (`$HOME/blis/include`).
-Also, note that `configure` will create any installation directories that do not already exist.
+The `--libdir=LIBDIR` and `--includedir=INCDIR` options will override any path implied by `PREFIX`, whether it was specified explicitly via `--prefix` or implicitly (via the default). That is, `LIBDIR` defaults to `EXECPREFIX/lib` (where `EXECPREFIX`, set via `--exec-prefix=EXECPREFIX`, defaults to `PREFIX`) and `INCDIR` defaults to `PREFIX/include`, but `LIBDIR` and `INCDIR` will each be overriden by their respective `--libdir`/`--includedir` options. There is a third related option, `--sharedir=SHAREDIR`, where `SHAREDIR` defaults to `PREFIX/share`. This option specifies the installation directory for certain makefile fragments that contain variables determined by `configure` (e.g. `CC`, `CFLAGS`, `LDFLAGS`, etc.). These files allow certain BLIS makefiles, such as those in the `examples` or `testsuite` directories, to operate on an installed copy of BLIS rather than a local (and possibly uninstalled) copy.
 
 For a complete list of supported `configure` options and arguments, run `configure` with the `-h` option:
 ```