Initial support for Haiku (#1045)

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

absl/base/config.h

diff --git a/absl/base/config.h b/absl/base/config.h
index 0562cdd..d3cad68 100644
--- a/absl/base/config.h
+++ b/absl/base/config.h
@@ -413,7 +413,8 @@
 #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