loader: load optional libraries with RTLD_LOCAL (JuliaLang#42631)

This prevents global lookups from finding symbols in optional
libraries, since that would only work "sometimes".

cli/loader_lib.c

@@ -41,10 +41,11 @@ static void * load_library(const char * rel_path, const char * src_dir, int err)
  break;
  basename++;
 #if defined(_OS_WINDOWS_)
- if ((handle = GetModuleHandleW(basename)))
+ if ((handle = GetModuleHandleA(basename)))
  return handle;
 #else
- if ((handle = dlopen(basename, RTLD_NOLOAD | RTLD_NOW | RTLD_GLOBAL)))
+ // if err == 0 the library is optional, so don't allow global lookups to see it
+ if ((handle = dlopen(basename, RTLD_NOLOAD | RTLD_NOW | (err ? RTLD_GLOBAL : RTLD_LOCAL))))
  return handle;
 #endif
 
@@ -61,7 +62,7 @@ static void * load_library(const char * rel_path, const char * src_dir, int err)
  }
  handle = (void *)LoadLibraryExW(wpath, NULL, LOAD_WITH_ALTERED_SEARCH_PATH);
 #else
- handle = dlopen(path, RTLD_NOW | RTLD_GLOBAL);
+ handle = dlopen(path, RTLD_NOW | (err ? RTLD_GLOBAL : RTLD_LOCAL));
 #endif
 
  if (handle == NULL) {