The BAZEL_BUILD macro is used in source; explicitly define in BUILD

Right now, this is only defined in the global `.bazelrc`, but that file is
not inspected if OpenROAD is used as submodule or dependency. So we need
to define all the relevant macros in the relevant BUILD rules.

Signed-off-by: Henner Zeller <h.zeller@acm.org>

Author: hzeller

src/tst/BUILD

@@ -41,6 +41,9 @@ cc_library(
     hdrs = [
         "include/tst/fixture.h",
     ],
+    defines = [
+        "BAZEL_BUILD",
+    ],
     includes = [
         "include",
     ],