pw_build: add "-Wundef" to strict_warnings

Adds "-Wundef" to "strict_warnings" to catch use of accidentally
undefined macros which otherwise implicitly default to 0.

Updates macros across Pigweed to make it compile.

Drops support for PW_TEST_DONT_DEFINE_* in addition to
GTEST_DONT_DEFINE_*, instead only GTEST_DONT_DEFINE_* is used.

Change-Id: Id89f85f7cb8a52c1619bc0f7262349310203067b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/21463
Reviewed-by: Alexei Frolov <frolv@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Ewout van Bekkum <ewout@google.com>
diff --git a/pw_build/BUILD.gn b/pw_build/BUILD.gn
index 26a7f50..339a031 100644
--- a/pw_build/BUILD.gn
+++ b/pw_build/BUILD.gn
@@ -72,6 +72,7 @@
     "-Wextra",
     "-Wimplicit-fallthrough",
     "-Wcast-qual",
+    "-Wundef",
 
     # Make all warnings errors, except for the exemptions below.
     "-Werror",