configure.ac: Don't link libgadu to everything

When AC_CHECK_LIB() is used, if no argument is
passed, it automatically puts the library into $LIBS [1]. This was
happening for libgadu, which should only be in the gg prpl.
This patch gives it enough for an argument to
stop this from happening.

[1] https://www.gnu.org/software/autoconf/manual/autoconf-2.66/html_node/Libraries.html