Autotools build system does not set OS_ defines

[ryandesign]

The cmake build system sets some custom defines for various OS types in CMakeLists.txt

if(${CMAKE_SYSTEM_NAME} MATCHES "Linux")
  add_definitions(-DOS_LINUX)
elseif(${CMAKE_SYSTEM_NAME} MATCHES "FreeBSD")
  add_definitions(-DOS_FREEBSD)
elseif(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
  add_definitions(-DOS_MACOSX)
endif()

The undocumented autotools build system does not. Perhaps it should?

[eustas]

TODO: add the following definitions to Bazel / naked make / autotools:

• OS_LINUX
• OS_FREEBSD
• OS_MACOSX
• BROTLI_HAVE_LOG2

[eustas]

Only Bazel left.