Makefile

Added a Linux/POSIX Makefile.

Using Clang rather than GCC on Linux/POSIX.
build.sh

Added a BASH script to the root directory which copies the Linux/POSIX build files to the root directory and passes through all command line arguments to GNU make.
data.mk

Added a Linux/POSIX version of data.mk.

Since Linux/POSIX typically come with a robust set of command line tools for performing ordinary tasks (in this case fetching files over HTTP and extracting files from an archive) the Linux/POSIX data.mk has rules to fetch and extract the Unicode Character Database, which the Windows version does not.
unicode.mk

Added a Linux/POSIX version of unicode.mk.
unicodedata.mk

Added a Linux/POSIX version of unicodedata.mk.
build.sh Permissions

Marked build.sh as executable.
constexpr Constructor

At Clang's insistence, added a constexpr constructor for NewlineT in the parsing/codeoutput.hpp.
Makefile Fix

Removed the include directive for test.mk -- which isn't and shouldn't be in the repository -- from the Linux/POSIX Makefile.

It should also be removed from the Windows Makefile.
Clang to GCC

Switched to building with GCC on Linux/POSIX.  Was having issues with Clang.  Will debug later.
Remove -fsanitize=address

Enabling address sanitizer was bloating build times -- specifically src/data/data.cpp (i.e. the UCD dump) -- to an unacceptable level, therefore it's now disabled.
.gitignore

The Linux/POSIX build system automatically downloads UCD.zip to the root directory and then extracts it into the UCD directory, meaning this file will be present despite the fact that it's not part of the repository.  Updated .gitignore accordingly.
Merge branch 'linux'

As the repository now builds on Linux, merging this branch.
.gitignore

Beginning work on implementing the Unicode Collation Algorithm (i.e. UCA).  Accordingly a copy of the Default Unicode Collation Element Table (i.e. DUCET) will be required.  Since the Unicode data files are not logically part of the repository, added a rule to .gitignore to ignore the UCA folder, where the DUCET data file (i.e. allkeys.txt) will reside.