Fix libFuzzer flag propagation for standalone builds

[george.karpenkov]

Summary

Under the previous configurations, flags from SANITIZER_COMMON were not propagated under some build settings.

[kcc]

LGTM

lib/fuzzer/CMakeLists.txt

 
 CHECK_CXX_SOURCE_COMPILES("
   static thread_local int blah;
   int main() {
   return 0;
   }
   " HAS_THREAD_LOCAL)
 
+set(LIBFUZZER_CFLAGS ${SANITIZER_COMMON_CFLAGS})
+
 if (CMAKE_CXX_FLAGS MATCHES "fsanitize-coverage")
-  set(LIBFUZZER_CFLAGS -fno-sanitize-coverage=trace-pc-guard,edge,trace-cmp,indirect-calls,8bit-counters)
+  list(APPEND LIBFUZZER_CFLAGS -fno-sanitize-coverage=trace-pc-guard,edge,trace-cmp,indirect-calls,8bit-counters)
 endif()
+
 if(NOT HAS_THREAD_LOCAL)
-  set(LIBFUZZER_CFLAGS "${LIBFUZZER_CFLAGS} -Dthread_local=__thread")
+  list(APPEND LIBFUZZER_CFLAGS -Dthread_local=__thread)
 endif()
 
 if(APPLE)
   set(FUZZER_SUPPORTED_OS osx)
 endif()
 
 add_compiler_rt_object_libraries(RTfuzzer
   OS ${FUZZER_SUPPORTED_OS}