Use backticks instead of non-portable make $(shell) function

The backticks are treated like text, until the shell expands them, thus
making this solution portable.

debian/changelog

@@ -79,6 +79,7 @@ dpkg (1.15.1) UNRELEASED; urgency=low
     Thanks to Philipp Kern <pkern@debian.org>. Closes: #526617
   * Add '.hgtags' to the default dpkg-source -i regex and -I pattern.
     Closes: #525854
+  * Use backticks instead of non-portable make $(shell) function in automake.
 
   [ Frank Lichtenheld ]
   * Dpkg::Version: Remove unnecessary function next_elem which just

man/Makefile.am

@@ -25,7 +25,7 @@ update-po:
 			./po4a.cfg 
 
 # Extract the list of languages from the po4a config file.
-LINGUAS = $(shell sed -ne 's/^.*\[po4a_langs\] \(.*\)$$/\1/p' $(srcdir)/po/po4a.cfg)
+LINGUAS = `sed -ne 's/^.*\[po4a_langs\] \(.*\)$$/\1/p' $(srcdir)/po/po4a.cfg`
 
 install-data-local: install-data-local-@USE_NLS@