Add system-wide configuration file and new config file environment vars

Add system-wide configuration file and new config file environment vars

Introduced envvars TIGRC_USER and TIGRC_SYSTEM to control which config
file is used. The default user specific config file can be overridden
with TIGRC_USER.  Before loading the user config file, tig now looks for
a system wide config file ($(sysconfdir)/tirgc by default). This can be
overridden with the environment variable TIGRC_SYSTEM.

Also corrected a small mistake in the Makefile. Instead of setting
CFLAGS for '-D...' compiler options, use CPPFLAGS.

[ The original code from Dominik was changed so that tig always reads
  the system-wide configuration file. The documentation was improved
  so the configured sysconfdir is used in the generated documentation.
  -- jonas ]

Signed-off-by: Jonas Fonseca <fonseca@diku.dk>