Use append_cflags over modifying CFLAGS directly (#355)

- Allows installing bson gem on systems with GCC 15
- Fixes https://jira.mongodb.org/browse/RUBY-3675

References:

- Ruby Lang - https://bugs.ruby-lang.org/issues/21290#61
- io-event gem
  - bug report: socketry/io-event#136
  - bugfix patch (+6 / -6): socketry/io-event@d085761
- ed25519 gem
  - bug report: RubyCrypto/ed25519#44
  - bugfix patch (+1 / -1): https://github.com/RubyCrypto/ed25519/pull/45/files#diff-805cf1ea9824de19b94dce429b3a3544fd9cb0b3cc278e571776c7d672cc1dfa
- jaro_winkler gem
  - bug report: tonytonyjan/jaro_winkler#61
  - bugfix patch (+1 / -1): https://github.com/tonytonyjan/jaro_winkler/pull/62/files#diff-594397402316a8262174f2dc8159b5a35560516bd3cd355b1bf657a6e0a973ce

Author: pboling

ext/bson/extconf.rb

@@ -3,6 +3,6 @@
 
 require 'mkmf'
 
-$CFLAGS << ' -Wall -g -std=c99'
+append_cflags(["-Wall", "-g", "-std=c99"])
 
 create_makefile('bson_native')