Add support for PCRE2

• Support and use PCRE2 by default
• Add pattern/regexp tests
• Update PCRE builtin tarball to its last version
• Various warning fixes
• Add EasyBuild doc and badge

Fix affinity "off" compilation issue

• Compilation failed when _GNU_SOURCE was not defined with affinity off
• Fix case on Mac for tests (and Tests) folders
• Use _SC_NPROCESSORS_ONLN instead of _SC_NPROCESSORS_CONF to detect the real number of available cores at runtime, not the maximal number of cores
• Harmonize bin/env paths
• Use PROJECT_SOURCE_DIR instead of PROJECT_BINARY_DIR for Tests/ folder in order to find it when installation is delocalized
• Fix broken links in doc