Update Make.inc to detect OpenBSD

With this patch I have been able to successfully compile Openlibm on OpenBSD 5.7 (x86_64) using the following command:

gmake GCC=egcc GXX=eg++ USEGCC=1 ARCH=x86_64

(I had to install gcc 4.9 from ports, as Openlibm does not compile with the default compiler shipped with OpenBSD 5.7, which is gcc 4.2.1).

Make.inc

@@ -103,6 +103,12 @@ SONAME_FLAG = -soname
 CFLAGS_add+=-fPIC
 endif
 
+ifeq ($(OS), OpenBSD)
+SHLIB_EXT = so
+SONAME_FLAG = -soname
+CFLAGS_add+=-fPIC
+endif
+
 ifeq ($(OS), Darwin)
 SHLIB_EXT = dylib
 SONAME_FLAG = -install_name