glsl/tests/optimisation-test: make sure that $PYTHON2 is set/available

Otherwise we'll fail when invoking the script outside of "make check"

v2: use -ne over a string comparison (Eric)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

src/compiler/glsl/tests/optimization-test.sh

@@ -6,6 +6,16 @@ else
    compare_ir=./compare_ir.py
 fi
 
+if [ -z "$PYTHON2" ]; then
+    PYTHON2=python2
+fi
+
+which $PYTHON2 >/dev/null
+if [ $? -ne 0 ]; then
+    echo "Could not find python2. Make sure that PYTHON2 variable is correctly set."
+    exit 1
+fi
+
 total=0
 pass=0