Special-case openbsd c++ libraries (#399)

* Special-case openbsd c++ libraries

They may be typical for clang-based systems, but I'm not sure what
that list is.

Fix #398

Related to https://gitlab.haskell.org/ghc/ghc/-/issues/20870

* Use libc++ on FreeBSD instead of libstdc++ which no longer exists

Author: blackgnezdo

text.cabal

@@ -95,11 +95,12 @@ library
         extra-libraries: stdc++-6 gcc_s_seh-1
       else
         extra-libraries: stdc++-6 gcc_s_dw2-1
+    elif os(darwin) || os(freebsd)
+      extra-libraries: c++
+    elif os(openbsd)
+      extra-libraries: c++ c++abi
     else
-      if os(darwin)
-        extra-libraries: c++
-      else
-        extra-libraries: stdc++
+      extra-libraries: stdc++
 
   exposed-modules:
     Data.Text