Replace non-portable `grep -q` with shell "case" in dejagnu launcher

ChangeLog

@@ -5,6 +5,9 @@
 	* dejagnu (command): Use Awk instead of non-portable basename(1)
 	and a non-portable sed(1) pattern to initially set this variable.
 
+	* dejagnu: Use shell "case" pattern match instead of non-portable
+	"grep -q" to determine if "awk" is GNU Awk.
+
 2021-04-15  Jacob Bachmeyer  <jcb@gnu.org>
 
 	PR47382

dejagnu

@@ -248,11 +248,10 @@ if $have_gawk ; then
 fi
 # is "awk" actually GNU Awk?
 if $have_awk ; then
-    if "$awkbin" --version | sed 1q | grep -qi 'GNU Awk' ; then
-	have_gawk_as_awk=true
-    else
-	have_gawk_as_awk=false
-    fi
+    case `"$awkbin" --version 2>&1 | sed 1q` in
+	*'GNU Awk'*)	have_gawk_as_awk=true	;;
+	*)		have_gawk_as_awk=false	;;
+    esac
 fi
 if expr "$verbose" \> 2 > /dev/null ; then
     if $have_awk ; then