* Move the libdl check to occur before the non-pkgconfig sqlite3 check,
and do it only once.
* Add dependency CFLAGS to CXXFLAGS, since there are now cpp files which
need them.
* Defined TSK_OPT_DEP_CHECK to eliminate duplicate code when checking for
optional dependencies.
* Added file for AX_PKG_CHECK_MODULES. Enabled use of pkg-config for
depenencies which are known to support it.
* Set library variable to "no" when AC_CHECK_HEADERS or AC_CHECK_LIB fails.