tests: add test for PKG_CONFIG_SYSROOT_DIR

tests/run.sh

@@ -88,6 +88,10 @@ run_test "PKG_CONFIG_PATH=${selfdir}/lib1 ${1} --variable=prefix foo" \
 run_test "PKG_CONFIG_PATH=${selfdir}/lib1 ${1} --define-variable=prefix=/test --variable=prefix foo" \
 	'/test'
 
+# 7) tests for env modifiers
+run_test "PKG_CONFIG_PATH=${selfdir}/lib1 PKG_CONFIG_SYSROOT_DIR=/test ${1} --cflags baz" \
+	'-I/test/usr/include/foo'
+
 if [ ${failed} -gt 0 ]; then
 	echo "${failed} of ${done} tests failed. See output for details." >&2
 	exit 1