problem: using -errwarn on non-Sun systems

solution: only use `-errwarn` on SunPro systems

For example, using Clang on Alpine, this just results in build spam, and
the option is incorrectly passed through to ld, which trys to use use it
(`-e`) as an entrypoint, i.e:
```bash
[ 28%] Building CXX object CMakeFiles/local_lat.dir/perf/local_lat.cpp.o
clang++: warning: -e rrwarn=%all: 'linker' input unused [-Wunused-command-line-argument]
[ 29%] Linking CXX executable bin/local_lat
/usr/bin/ld: warning: cannot find entry symbol rrwarn=%all; defaulting to 0000000000000cf0
[ 29%] Built target local_lat
```

CMakeLists.txt

@@ -705,7 +705,7 @@ if(LIBZMQ_WERROR)
     zmq_check_cxx_flag_prepend("/WX")
   else()
     zmq_check_cxx_flag_prepend("-Werror")
-    if(NOT "${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
+    if(${CMAKE_CXX_COMPILER_ID} MATCHES "SunPro")
       zmq_check_cxx_flag_prepend("-errwarn=%all")
     endif()
   endif()