Use abseil-cpp VLOG (#354)

Abseil-cpp LTS 20240116 now supports VLOG.

https://github.com/abseil/abseil-cpp/releases/tag/20240116.1

Use it instead of making VLOG a no-op.

CMakeLists.txt

@@ -57,6 +57,10 @@ find_package(OpenSSL REQUIRED)
 # pthreads isn't used directly, but this is still required for std::thread.
 find_package(Threads REQUIRED)
 
+if (NOT TARGET absl::vlog_is_on)
+    message(FATAL_ERROR "Could not find absl vlog module. Are you using an older version?")
+endif()
+
 if (WITH_PYTHON)
     # Should be easy to make it work with swig3, but some args to %pythonprepend
     # seem to be different and were changed.
@@ -227,6 +231,7 @@ target_link_libraries(
     absl::strings
     absl::type_traits
     absl::utility
+    absl::vlog_is_on
     ${CMAKE_THREAD_LIBS_INIT})
 
 if (GOOGLETEST_ROOT)

README.md

@@ -90,7 +90,7 @@ cd s2geometry
 
 ### Building
 
-First, [install Abseil](https://github.com/abseil/abseil-cpp/blob/master/CMake/README.md#traditional-cmake-set-up).
+First, [install Abseil](https://github.com/abseil/abseil-cpp/blob/master/CMake/README.md#traditional-cmake-set-up). We require [20240116.1](https://github.com/abseil/abseil-cpp/releases/tag/20240116.1) as the minimal version.
 It must be configured with `-DCMAKE_POSITION_INDEPENDENT_CODE=ON`.
 s2geometry must be configured to use the same C++ version that
 abseil uses.  The easiest way to achieve this is to pass

src/s2/base/logging.h

@@ -20,6 +20,7 @@
 // include the relevant absl file directly instead.
 #include "absl/log/check.h"
 #include "absl/log/log.h"
+#include "absl/log/vlog_is_on.h"
 #include "s2/base/log_severity.h"
 
 // The names CHECK, etc. are too common and may conflict with other
@@ -48,14 +49,7 @@
 #define S2_DCHECK_GT DCHECK_GT
 #define S2_DCHECK_GE DCHECK_GE
 
-// Logging stream that does nothing.
-struct S2NullStream {
-  template <typename T>
-  S2NullStream& operator<<(const T& v) { return *this; }
-};
-
-// Abseil-cpp doesn't support VLOG yet.  Make VLOG a no-op.
-#define S2_VLOG(verbose_level) S2NullStream()
-#define S2_VLOG_IS_ON(verbose_level) (false)
+#define S2_VLOG VLOG
+#define S2_VLOG_IS_ON VLOG_IS_ON
 
 #endif  // S2_BASE_LOGGING_H_