CMake: don't check for --no-undefined linker flag for OpenBSD (bug #5174)

it now matches autotools.

CMakeLists.txt

@@ -528,7 +528,7 @@ if(USE_GCC OR USE_CLANG)
     list(APPEND EXTRA_LDFLAGS "-Wl,-undefined,error")
     list(APPEND EXTRA_LDFLAGS "-Wl,-compatibility_version,${DYLIB_COMPATIBILITY_VERSION}")
     list(APPEND EXTRA_LDFLAGS "-Wl,-current_version,${DYLIB_CURRENT_VERSION}")
-  else()
+  elseif(NOT OPENBSD)
     set(CMAKE_REQUIRED_FLAGS "-Wl,--no-undefined")
     check_c_compiler_flag("" HAVE_NO_UNDEFINED)
     set(CMAKE_REQUIRED_FLAGS ${ORIG_CMAKE_REQUIRED_FLAGS})