--with-<package>* issue

[bosilca]

Describe the bug
I'm trying to use OAC_CHECK_PACKAGE to find support for JIT in gcc. The header (libgccjit.h) and the necessary library (-lgccjit) being installed in totally non-standard locations (with no common root) I need to use both --with-<package>-incdir and --with-<package>-libdir to somewhat get things working.

Unfortunately, adding --with-<package> without specifying a path, prevent the correct discovery because of the _OAC_CHECK_PACKAGE_GENERICmacro. This macro requires check_package_prefix (which is only set if --with-<package> has a path), in order to do the package lookup or it assumes standard locations for the header and library.

I think AS_IF([test -n "${check_package_prefix}"], in _OAC_CHECK_PACKAGE_GENERIC should be changed to a more lenient test, one that encompass also the existence of check_package_incdir and check_package_libdir as it is done in the _OAC_CHECK_PACKAGE_GENERIC_PREFIX macro.