Improve test for clang presence

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index 8adf3f4..28767eb 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -4340,7 +4340,7 @@
 }
 
 support_build_sha_armce() {
-    if ( $(which clang > /dev/null) ); then
+    if command -v clang > /dev/null ; then
         # clang >= 4 is required to build with SHA extensions
         clang_ver="$(clang --version|grep version|sed -E 's#.*version ([0-9]+).*#\1#')"