[MacPorts] #38825: gtkglextmm @1.2.0: fix underquoted macro warning

[MacPorts]

#38825: gtkglextmm @1.2.0: fix underquoted macro warning
---------------------+--------------------------------
 Reporter:  egall@…  |      Owner:  macports-tickets@…
     Type:  defect   |     Status:  new
 Priority:  Normal   |  Milestone:
Component:  ports    |    Version:  2.1.3
 Keywords:           |       Port:  gtkglextmm
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 {{{
 /opt/local/share/aclocal/gtkglextmm-1.2.m4:225: warning: underquoted
 definition of AC_GTKGLEXTMM_SUPPORTS_MULTIHEAD
 /opt/local/share/aclocal/gtkglextmm-1.2.m4:225:   run info Automake
 'Extending aclocal'
 /opt/local/share/aclocal/gtkglextmm-1.2.m4:225:   or see
 http://www.gnu.org/software/automake/manual/automake.html#Extending-
 aclocal
 }}}
 {{{
 Local-Admins-MacBook-Pro:~ ericgallager$ port provides
 /opt/local/share/aclocal/gtkglextmm-1.2.m4
 /opt/local/share/aclocal/gtkglextmm-1.2.m4 is provided by: gtkglextmm
 }}}
 This is just like #38796

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Ticket URL: <https://trac.macports.org/ticket/38825>
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