Build: configure: Use AC_SEARCH_LIBS for basic libraries.

• Get rid of the check for getopt_long, which is no longer used anywhere.

• Use AC_SEARCH_LIBS for finding libdl, but because that macro always adds the result of the test to $LIBS, we have to use a trick: save $LIBS, search for libdl, set $DL_LIBS to the result, and then restore $LIBS.

• Change LIBADD_DL to DL_LIBS for future consistency.

• Get rid of the "Where is dlopen?" block. I think AC_SEARCH_LIBS makes this unnecessary.

• libcrmcommon links against socket and is used by everything, so we can just simply call AC_SEARCH_LIBS for it.

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