Fix false libtool variable: _LIBS -> _LIBADD

_LIBS is not a libtool variable, thus the remotebackend module does not
get linked to libcurl. Fix this by using _LIBADD instead.
Use LUA_CFLAGS and LUA_LIBS instead of hardcoded paths.
Restructure system PolarSSL detection

Based on the chapter "Automatic Dependencies with AC_ARG_WITH" of the
Autotools Mythbuster eBook, fix the system PolarSSL detection and
recognize both --with/without-system-polarssl properly. Furthermore,
set the HAVE_LIBPOLARSSL automake variable conditionally. Otherwise pdns will
use the included headers but link against the system polarssl resulting in
subtle breaks.
Replace PolarSSL-conditionals in Makefile.am and trust the include dirs

Different ways of using the bundled PolarSSL have been used (-L.. vs
if/then/else). Replace this by defining POLARSSL_CFLAGS/_LIBS/_SUBDIR in
configure.ac and use that in all makefiles. Remove includes based on the
preprocessor HAVE_LIBPOLARSSL definition and trust the CFLAGS set in the
environment to point to the correct directory for the PolarSSL include.
Merge pull request #825 from dev-zero/master

Various build system fixes (3rd try)