Rakefile: improve filtering optimization flags for iso639_language_list.cpp

The old regular expression did not filter out non-numeric optimization
levels options such as `-Os`, nor did it filter the option out if it
was at the end of the flags string.

See 3105 for context.

NEWS.md

@@ -1,3 +1,13 @@
+# Version ?
+
+## Bug fixes
+
+* build system: fixed filtering out optimization options when compiling the
+  file `iso639_language_list.cpp` (before only numeric optimization levels
+  were filtered out and only if it wasn't the last option in the list of
+  flags). See #3105 for context.
+
+
 # Version 57.0.0 "Till The End" 2021-05-22
 
 ## New features and enhancements

Rakefile

@@ -355,7 +355,7 @@ cxx_compiler = lambda do |*args|
     # Even -O1 causes compilation time & memory usage to skyrocket,
     # possibly exponentially, with the number of entries to
     # emplace_back() into the vector.
-    flags.gsub!(%r{ -O\d+ }, ' -O0 ')
+    flags.gsub!(%r{(^| )-O[^ ]*}, ' -O0')
   end
 
   args = [