Update to improve build system & unit tests, & clean up a bunch of compile & static analysis warnings.

• Buildsys: 'install-strip', 'uninstall', 'check', and 'distcheck' make targets now do useful things. (the install-strip change addresses the remaining bit of issue #18.)
• Buildsys: To support old flavors of gcc, add "--disable-extrawarnings" configure option for compilers that don't understand -Wextra arg, and detect if the local g++ happens to be called c++.
• Fix various new warnings w/ GCC 7.
• Use std::unique_ptr instead of deprecated std::auto_ptr, where available.
• Clean up / enable more twtest unit tests, & tweak the framework for clarity.
• Remove more dead code and strip out a bunch of uses of the always-true IS_UNIX ifdef.
• Address some static analysis warnings from CppCheck & Flawfinder
• Add a sanity check in fcodatasourceiterimpl to address a unit test that failed on some platforms.
• Update old policy test files in src/parser/testfiles to use current policy syntax.