Build the compiler on OpenBSD.

Index: src/include/a68g.h
--- src/include/a68g.h.orig
+++ src/include/a68g.h
@@ -32,7 +32,7 @@
 
 // Build switches depending on platform.
 
-#if (defined (BUILD_LINUX) && defined (HAVE_GCC) && defined (HAVE_DL))
+#if (defined (HAVE_GCC))
 #  define BUILD_A68_COMPILER
 #else
 // Untested, so disabled.
