After a recent OSX upgrade it seems that i can no longer build bson_ext locally

I'm running OSX 10.15.6

```

cbson.c:602:18: error: implicit declaration of function 'bson_buffer_get_max_size' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
if (length > bson_buffer_get_max_size(buffer)) {
^
cbson.c:602:18: note: did you mean 'bson_buffer_set_max_size'?
./bson_buffer.h:34:6: note: 'bson_buffer_set_max_size' declared here
void bson_buffer_set_max_size(bson_buffer_t buffer, int max_size);
^
6 warnings and 1 error generated.
make: *** [cbson.o] Error 1

make failed, exit code 2

Gem files will remain installed in /Users/rsim/.asdf/installs/ruby/2.6.6/lib/ruby/gems/2.6.0/gems/bson_ext-1.5.1 for inspection.
Results logged to /Users/rsim/.asdf/installs/ruby/2.6.6/lib/ruby/gems/2.6.0/extensions/x86_64-darwin-19/2.6.0/bson_ext-1.5.1/gem_make.out

An error occurred while installing bson_ext (1.5.1), and Bundler cannot continue.
Make sure that `gem install bson_ext -v '1.5.1'` succeeds before bundling.

```

```

$ gcc --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/4.2.1
Apple clang version 12.0.0 (clang-1200.0.32.2)
Target: x86_64-apple-darwin19.6.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

```