Fix error: use of undeclared identifier 'ABSL_INTERNAL_ASSUME' (#238)

`ABSL_INTERNAL_ASSUME` was replaced by `ABSL_ASSUME` in
abseil/abseil-cpp@231c393.

Define `S2_ASSUME` to either `ABSL_ASSUME` or `ABSL_INTERNAL_ASSUME`, depending on what's available. The latest LTS release does not yet have `S2_ASSUME`.

Fixes #237

Author: t-brito

AUTHORS

@@ -11,3 +11,4 @@
 Dan Larkin-York <dan@arangodb.com>
 Google Inc.
 Koordinates Limited
+Tiago Brito <tiago.brito@90poe.io>

CONTRIBUTORS

@@ -28,3 +28,4 @@ Jesse Rosenstock <jmr@google.com>
 Julien Basch <julienbasch@google.com>
 Phil Elson <pelson.pub@gmail.com>
 Robert Coup <robert.coup@koordinates.com>
+Tiago Brito <tiago.brito@90poe.io>

src/s2/util/bits/bits.h

@@ -59,6 +59,15 @@
 #include <intrin.h>
 #endif
 
+// TODO: Remove this and just use ABSL_ASSUME when it is in an LTS release.
+#if defined(ABSL_ASSUME)
+#define S2_ASSUME(cond) ABSL_ASSUME(cond)
+#elif defined(ABSL_INTERNAL_ASSUME)
+#define S2_ASSUME(cond) ABSL_INTERNAL_ASSUME(cond)
+#else
+#error "abseil-cpp must provide ABSL_ASSUME or ABSL_INTERNAL_ASSUME, what version are you using?"
+#endif
+
 #include <type_traits>
 
 #include "s2/base/integral_types.h"
@@ -362,7 +371,7 @@ inline int Bits::Log2Floor(T &&n) {
 }
 
 inline int Bits::Log2FloorNonZero(uint32 n) {
-  ABSL_INTERNAL_ASSUME(n != 0);
+  S2_ASSUME(n != 0);
   return absl::bit_width(n) - 1;
 }
 
@@ -384,17 +393,17 @@ inline int Bits::Log2Floor64(T &&n) {
 }
 
 inline int Bits::Log2FloorNonZero64(uint64 n) {
-  ABSL_INTERNAL_ASSUME(n != 0);
+  S2_ASSUME(n != 0);
   return absl::bit_width(n) - 1;
 }
 
 inline int Bits::FindLSBSetNonZero(uint32 n) {
-  ABSL_INTERNAL_ASSUME(n != 0);
+  S2_ASSUME(n != 0);
   return absl::countr_zero(n);
 }
 
 inline int Bits::FindLSBSetNonZero64(uint64 n) {
-  ABSL_INTERNAL_ASSUME(n != 0);
+  S2_ASSUME(n != 0);
   return absl::countr_zero(n);
 }