gl_GLIBC21 and charset.alias

[John Ralls <jralls ceridwen us>]

In https://git.gnome.org/browse/glib/commit/?id=d2bb019533bbcbd1cee3891822ac7ba6ff55ad3d , Colin removed the 
call to gl_GLIBC21 from configure.ac, which breaks the installation of charset.alias because clause in 
glib/libcharset/Makefile.am which installs it depends upon @GLIBC21@ which is no longer defined.

My inclination is to just remove the condition, since charset.alias gets built anyway; the test only stops it 
being installed. Is there any harm in having charset.alias installed if the system has glibc-2.1 or later? If 
so, is there something to test for that's preferable over the gl_GLIBC21 autoconf macro?

Regards,
John Ralls