switch from pcre to pcre2

[remicollet]

libpcre is deprecated, libpcre2 should be preferred.

Upcoming RHEL-10 won't have libpcre

Notice: cmake stuff not done, only autotools

[remicollet]

Tested on RHEL-10-0-Beta using 1.0.0 + this patch

============================================================================
Testsuite summary for handlebars 1.0.0
============================================================================
# TOTAL: 1939
# PASS:  1939
# SKIP:  0
# XFAIL: 0
# FAIL:  0
# XPASS: 0
# ERROR: 0
============================================================================

And for PHP extension

=====================================================================
TEST RESULT SUMMARY
---------------------------------------------------------------------
Exts skipped    :     0
Exts tested     :    40
---------------------------------------------------------------------

Number of tests :  1398              1326
Tests skipped   :    72 (  5.2%) --------
Tests warned    :     0 (  0.0%) (  0.0%)
Tests failed    :     0 (  0.0%) (  0.0%)
Tests passed    :  1326 ( 94.8%) (100.0%)
---------------------------------------------------------------------
Time taken      : 0.782 seconds
=====================================================================