[BUILD] disable warning about localtime_r on Solaris

Solaris only declares localtime_r() if _REENTRANT is defined. Update
the makefile to silent the warning.
diff --git a/Makefile b/Makefile
index e0aca9b..5fa8622 100644
--- a/Makefile
+++ b/Makefile
@@ -69,7 +69,7 @@
 LIBS.linux22 =
 
 # This is for Solaris 8
-COPTS.solaris = -fomit-frame-pointer -DFD_SETSIZE=65536
+COPTS.solaris = -fomit-frame-pointer -DFD_SETSIZE=65536 -D_REENTRANT
 LIBS.solaris = -lnsl -lsocket
 
 # CPU dependant optimizations