gnu: pcre: Update to 8.36 and enable JIT support.

* gnu/packages/pcre.scm (pcre): Update to 8.36.  Enable JIT support.
gnu: Add nginx.

* gnu/packages/web.scm (nginx): New variable.
system: /etc/profile defines 'MANPATH', which includes the global profile.

* gnu/system.scm (etc-directory)[profile]: Set 'MANPATH'.
gnu: Add samtools

* gnu/packages/bioinformatics.scm: New file
* gnu-system.am (GNU_SYSTEM_MODULES): Add it

Signed-off-by: Ludovic Courtès <ludo@gnu.org>
gnu: xfconf: Disable parallel tests.

* gnu/packages/xfce.scm (xfconf): Add 'arguments' field.
gnu: python-numpy-bootstrap: Make tests verbose.

* gnu/packages/python.scm (python-numpy-bootstrap): Make tests verbose to
  allow test failure debugging on hydra.