Problem: some systems can't find pcre.h

Solution: help by pre-populating a CPPFLAGS value

Signed-off-by: Jim Klimov <EvgenyKlimov@eaton.com>

Problem: no test for issue #11

Solution: add teststr.gsl to make check and add non-matching search_replace to test

See #11

Problem: memory leak if string.search_replace has nothing to replace

Solution: use mem_strdup instead of strdup

Fixes #11

Problem: Build can't always reasonable run binary

Solution: Build the test stage instead, since that's where
the ./gsl -v build was moved

Solution: Put the ./gsl -v  call in check: and make check: depend
on everything: to make it easy to see in Travis.

Problem: gcc is not always called gcc

Solution: Query the --version string of whatever CCNAME happens to
be. This will catch such misfits as cross compilers, and places
where gcc is invoked as plain old cc.

Problems: more improvements needed for debian packaging

Solution: list them under BSD-3-Clause

Solution: remove all mentions of pcre/*

Solution: add them

Solution: further simplify it using the .install files

Solution: run dh with --parallel

Solution: further simplify it

Problem: old and bugged PCRE version embedded

Solution: remove it. All supported builds work with a system distributed
version of PCRE, so it's not necessary anymore.
Fixes #1