Direct port as a shared library of a subset of Perl RE

To build
  smake -> build library
  smake test -> build unit_test

https://github.com/cesanta/slre

 * Copyright (c) 2004-2013 Sergey Lyubka <valenok@gmail.com>
 * Copyright (c) 2013 Cesanta Software Limited
 * All rights reserved
 *
 * This library is dual-licensed: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation. For the terms of this
 * license, see <http://www.gnu.org/licenses/>.
 *
 * You are free to use this library under the terms of the GNU General
 * Public License, but WITHOUT ANY WARRANTY; without even the implied
 * warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 * See the GNU General Public License for more details.
 *
 * Alternatively, you can license this library under a commercial
 * license, as set out in <http://cesanta.com/products.html>.

PERMISSION  UID  GID    PACKED    SIZE  RATIO METHOD CRC     STAMP     NAME
---------- ----------- ------- ------- ------ ---------- ------------ ----------
[unknown]                  529    1613  32.8% -lh5- 8a4e Apr 10 10:20 Makefile
[unknown]                 4079   13983  29.2% -lh5- 84bb Apr  8 18:48 slre.c
[unknown]                  923    1867  49.4% -lh5- cf9c Apr 10 10:23 slre.h
[unknown]                   99     108  91.7% -lh5- aa9e Apr  8 18:43 slre_lib.fd
[unknown]                   48      48 100.0% -lh0- 9b51 Apr  8 19:21 slre_pragmas.h
[unknown]                  166     357  46.5% -lh5- b617 Apr  8 19:09 slre_protos.h
[unknown]                 3008   12901  23.3% -lh5- fbe2 Apr 10 10:23 unit_test.c
[unknown]                  708    1485  47.7% -lh5- dc45 Oct 26  2020 docs/API.MD
[unknown]                   86     120  71.7% -lh5- 567a Oct 26  2020 docs/EXAMPLES.MD
[unknown]                  365     582  62.7% -lh5- 4579 Oct 26  2020 docs/find-urls.md
[unknown]                  152     284  53.5% -lh5- ecef Oct 26  2020 docs/INTRO.MD
[unknown]                  407     710  57.3% -lh5- 5cc8 Oct 26  2020 docs/LICENSE.MD
[unknown]                  484     748  64.7% -lh5- 5214 Oct 26  2020 docs/OVERVIEW.MD
[unknown]                  254     427  59.5% -lh5- 2a8e Oct 26  2020 docs/parse-http-request.md
[unknown]                  408     948  43.0% -lh5- c25e Oct 26  2020 docs/SYNTAX.MD
[unknown]                  923    1867  49.4% -lh5- cf9c Apr 10 10:23 library/slre.h
[unknown]                 3111    5160  60.3% -lh5- 8b72 Apr 10 10:24 library/slre.library
[unknown]                   48      48 100.0% -lh0- 9b51 Apr  8 19:21 library/slre_pragmas.h
---------- ----------- ------- ------- ------ ---------- ------------ ----------
 Total        18 files   15798   43256  36.5%            Apr 11 00:32