Introduce SNAPPY_ATTRIBUTE_ALWAYS_INLINE.

An internal CL started using ABSL_ATTRIBUTE_ALWAYS_INLINE
from Abseil. This CL introduces equivalent functionality as
SNAPPY_ALWAYS_INLINE.

PiperOrigin-RevId: 306289650

CMakeLists.txt

@@ -103,6 +103,13 @@ int main() {
   return __builtin_ctzll(0);
 }" HAVE_BUILTIN_CTZ)
 
+check_cxx_source_compiles("
+__attribute__((always_inline)) int zero() { return 0; }
+
+int main() {
+  return zero();
+}" HAVE_ATTRIBUTE_ALWAYS_INLINE)
+
 check_cxx_source_compiles("
 #include <tmmintrin.h>