main: Mangle required gettext version

Without this, when building libgtop, autopoint will look at configure.ac, and see:

AM_GNU_GETTEXT_VERSION([0.19.6])

and unpack the 0.19.6 version of the macros from:

/usr/share/gettext/archive.dir.tar.xz

But when compiling, will throw:

*** error: gettext infrastructure mismatch: using a Makefile.in.in from gettext version 0.19 but the autoconf macros are from gettext version 0.20

Because version 0.20 is the compat version of what we're using, gettext 0.21.