Revert "Re-implement macro to constexpr transition"

This reverts commit 3d8f98c395.

In addition to the issues mentioned on the GitHub page for this commit,
it also broke the CentOS 7 build.

Note one can locally test the CentOS 7 build via:

    ./docker/docker_run_tests.sh ./docker/centos7.Dockerfile

src/autoload.cpp

@@ -17,7 +17,7 @@
 #include "wutil.h"  // IWYU pragma: keep
 
 /// The time before we'll recheck an autoloaded file.
-const int kAutoloadStalenessInterval = 15;
+static const int kAutoloadStalenessInterval = 15;
 
 /// Represents a file that we might want to autoload.
 namespace {