cmake: Fix pot-update target breakage when bash isn't the default shell

(Default shell as in the /bin/sh executable.)

On Debian and other systems using a different Bourne-compatible shell
that doesn't support some of Bash's syntax extensions (in particular,
'&>' as a redirection operator targeting stdout+stderr), the pot-update
target is completely broken, stuffing the POTFILES.in lists for either
the last (wesnoth-utbs) or first (wesnoth-ai) textdomains with paths for
every C/C++ source file from our tree.

Shells that do not interpret &> the way Bash does consider it to be the
stdout redirection operator following an asynchronous invocation. Of
course such a thing is bound to cause weird issues like this.

changelog

@@ -131,6 +131,8 @@ Version 1.13.0-dev:
    * Updated translations: Czech, French, Galician, German, Greek, Hungarian,
      Italian, Lithuanian, Portuguese, Russian, Scottish Gaelic, Slovak, Spanish,
      Vietnamese
+   * Fixed cmake pot-update target on Debian and other systems that do not use
+     bash as the default /bin/sh.
  * Lua API:
    * Upgraded Lua to version 5.2.3.
    * Fix bug #21761: wesnoth.synchronize_choice will now give a warning when

cmake/update_pot_source_dependencies.cmake

@@ -31,7 +31,7 @@ if(DOMAIN STREQUAL ${DEFAULT_DOMAIN})
 					# If the file doesn't contain a GETTEXT_DOMAIN
 					# definition it should be added to the default domain.
 					if ! grep \"^\#define  *GETTEXT_DOMAIN\" 
-							$$file &>/dev/null\; then 
+							$$file > /dev/null 2>&1\; then
 
 						echo $$file >> 
 							${PROJECT_SOURCE_DIR}/po/${DOMAIN}/POTFILES.in \;
@@ -61,7 +61,7 @@ else(DOMAIN STREQUAL ${DEFAULT_DOMAIN})
 					# If the file contains a GETTEXT_DOMAIN definition for
 					# the current domain add it to the domain.
 					if grep \"^\#define  *GETTEXT_DOMAIN  *\\\"${DOMAIN}\\\"\"
-							$$file &>/dev/null\; then 
+							$$file > /dev/null 2>&1\; then
 
 						echo $$file >> 
 							${PROJECT_SOURCE_DIR}/po/${DOMAIN}/POTFILES.in \;