Allow use of old zlib-1.2.3 on OpenBSD.

As of 2020-08-01, OpenBSD's system zlib is 1.2.3 which doesn't
have gzoffset(). However we can get away with this by making
gzfilebuf::approxOffset() always return zero.

CMakeLists.txt

@@ -278,7 +278,14 @@ else()
     endif()
 endif(SIMGEAR_HEADLESS)
 
-find_package(ZLIB 1.2.4 REQUIRED)
+if(${CMAKE_SYSTEM_NAME} MATCHES "OpenBSD")
+    # As of 2020-08-01, OpenBSD's system zlib is slightly old, but it's usable
+    # with a workaround in simgear/io/iostreams/gzfstream.cxx.
+    find_package(ZLIB 1.2.3 REQUIRED)
+else()
+    find_package(ZLIB 1.2.4 REQUIRED)
+endif()
+
 find_package(CURL REQUIRED)
 
 if (SYSTEM_EXPAT)