support/dependencies: unbreak check for UTF-8 locale

Although the UTF-8 locales in mainstream distributions all are suffixed
with just 'utf8', the nomenclature is a bit ambiguous with the way they
are to be specified with the various LC_* variables, suffixed there with
'UTF-8'.

Also, POSIX, ISO, and IEC do not enforce any specific suffix in LC_*
variables:
    http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_02



    """
    If the locale value has the form:
        language[_territory][.codeset]

    it refers to an implementation-provided locale, where settings of
    language, territory, and codeset are implementation-defined.
    """

To avoid any confusion, use a regexp that is a bit more lax when
matching locales.

Also, quote the regexp, so that the '?' and '$' are not interpreted by
the shell.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>