- default to OpenBSD detection (still overridable with OSTYPE env)

Index: src/fpm_environment.f90
--- src/fpm_environment.f90.orig
+++ src/fpm_environment.f90
@@ -114,6 +114,11 @@ contains
             end if
         end if
 
+        ! We are in ports tree, it is OpenBSD
+        r = OS_OPENBSD
+        ret = r
+        return
+
         ! Linux
         inquire (file='/etc/os-release', exist=file_exists)