d5b523e renamed the parser functions to include a thin_ prefix as a namespace, but did not update the parser header file. This did not cause any problems for a long time because implicit-function-declaration was just a warning previously, but Xcode 12 has made it an error (second bullet under Apple Clang Compiler > Resolved):

Clang now reports an error when you use a function without an explicit declaration when building C or Objective-C code for macOS (-Werror=implicit-function-declaration flag is on). This additional error detection unifies Clang’s behavior for iOS/tvOS and macOS 64-bit targets for this diagnostic. (49917738)

This is preventing install of this gem on macOS with Xcode 12.

Note: there is a workaround until this is patched:

$ gem install thin -- --with-cflags="-Wno-error=implicit-function-declaration"

or with Bundler:

$ bundle config build.thin --with-cflags="-Wno-error=implicit-function-declaration"