We unconditionally appended ">= $min_glib_version" to the modules to
look for, even though we had already included
"glib-2.0 >= $min_glib_version" in our list. When requesting additional
modules, this was fine, for example

    AM_PATH_GLIB_2_0([2.58], [:], [:], [gobject gio])

ended up asking pkg-config for

    glib-2.0 >= 2.58 gobject-2.0 gio-2.0 >= 2.58

which is redundant (since they all share a version number) but
otherwise OK.

However,

    AM_PATH_GLIB_2_0([2.58], [:], [:], [])

ended up asking pkg-config for

    glib-2.0 >= 2.58 >= 2.58

which is not OK; the second ">=" was parsed as a bizarrely-named package
to check for, and obviously few people have ">=.pc" installed.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Fixes: 4bb16f48 "m4macros: Allow information from pkg-config to be overridden"