Fix extconf.rb to look for fdatasync in unistd.h

Fix: #470

Author: byroot

CHANGELOG.md

@@ -1,5 +1,7 @@
 # Unreleased
 
+* Fix a potential compilation issue on some systems. See #470.
+
 # 1.18.1
 
 * Handle `EPERM` errors when opening files with `O_NOATIME`.

ext/bootsnap/bootsnap.c

@@ -18,6 +18,7 @@
 #include <sys/types.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <unistd.h>
 #include <sys/stat.h>
 
 #ifndef O_NOATIME

ext/bootsnap/extconf.rb

@@ -3,7 +3,7 @@
 require "mkmf"
 
 if %w[ruby truffleruby].include?(RUBY_ENGINE)
-  have_func "fdatasync", "fcntl.h"
+  have_func "fdatasync", "unistd.h"
 
   unless RUBY_PLATFORM.match?(/mswin|mingw|cygwin/)
     append_cppflags ["_GNU_SOURCE"] # Needed of O_NOATIME