m4macros: Fix detection of glib-2.0 alone

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"
9 jobs for <span class="ref-name">1543-glib-2-58-backports</span> in 11 minutes and 38 seconds (queued for 2 minutes and 3 seconds)
Status Job ID Name Coverage
  Build
passed #123520
cross-android_api21_arm64

00:01:31

passed #123521
cross-android_api28_arm64

00:00:55

passed #123522
cross-mingw64

00:02:09

passed #123519
fedora-autotools-x86_64

00:04:44

passed #123518
fedora-x86_64

00:04:02

manual #123525
freebsd-11 allowed to fail manual
freebsd-11-x86_64
passed #123523
win32
msys2-mingw32

00:08:00

passed #123524
win32
vs2017-x64

00:07:26

 
  Coverage
passed #123526
coverage

00:01:18

74.5%