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)

simgear/io/iostreams/gzfstream.cxx

@@ -185,6 +185,9 @@ gzfilebuf::setcompressionstrategy( int comp_strategy )
 
 z_off_t
 gzfilebuf::approxOffset() {
+    #ifdef __OpenBSD__
+    z_off_t res = 0;
+    #else
     z_off_t res = gzoffset(file);
 
     if (res == -1) {
@@ -201,7 +204,7 @@ gzfilebuf::approxOffset() {
         SG_LOG( SG_GENERAL, SG_ALERT, errMsg );
         throw sg_io_exception(errMsg);
     }
-
+    #endif
     return res;
 }