BUILD: makefile: Enable closefrom() support on Solaris

Solaris 9 (released 2002) added support for closefrom().

I bumped the version in the comment to 10 as the default feature
flags already has event ports enabled which were introduced in
Solaris 10.
diff --git a/INSTALL b/INSTALL
index 69fe94f..04b0349 100644
--- a/INSTALL
+++ b/INSTALL
@@ -374,7 +374,7 @@
   - linux-glibc         for Linux kernel 2.6.28 and above
   - linux-glibc-legacy  for Linux kernel 2.6.28 and above without new features
   - linux-musl          for Linux kernel 2.6.28 and above with musl libc
-  - solaris             for Solaris 8 or 10 (others untested)
+  - solaris             for Solaris 10 and above
   - freebsd             for FreeBSD 5 to 12 (others untested)
   - netbsd              for NetBSD
   - osx                 for Mac OS/X