When krb5-config was found, MUTT_AM_PATH_GSSAPI included CFLAGS in
GSSAPI_CFLAGS and MUTTLIBS in GSSAPI_LIBS.  However, configure.ac
afterwards sets:
    CPPFLAGS="$CPPFLAGS $GSSAPI_CFLAGS"
    MUTTLIBS="$MUTTLIBS $GSSAPI_LIBS"
This caused the flags and libs to be duplicated.