Initial support for Haiku (abseil#1045)

This provides the baseline needed to build Abseil on Haiku systems.

Author: korli

absl/base/config.h

@@ -413,7 +413,8 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 #elif defined(__linux__) || defined(__APPLE__) || defined(__FreeBSD__) || \
     defined(_AIX) || defined(__ros__) || defined(__native_client__) ||    \
     defined(__asmjs__) || defined(__wasm__) || defined(__Fuchsia__) ||    \
-    defined(__sun) || defined(__ASYLO__) || defined(__myriad2__)
+    defined(__sun) || defined(__ASYLO__) || defined(__myriad2__) ||       \
+    defined(__HAIKU__)
 #define ABSL_HAVE_MMAP 1
 #endif