- In base.h, if BORINGSSL_PREFIX is defined, include
  boringssl_prefix_symbols.h
- In all .S files, if BORINGSSL_PREFIX is defined, include
  boringssl_prefix_symbols_asm.h
- In base.h, BSSL_NAMESPACE_BEGIN and BSSL_NAMESPACE_END are
  defined with appropriate values depending on whether
  BORINGSSL_PREFIX is defined; these macros are used in place
  of 'namespace bssl {' and '}'
- Add util/make_prefix_headers.go, which takes a list of symbols
  and auto-generates the header files mentioned above
- In CMakeLists.txt, if BORINGSSL_PREFIX and BORINGSSL_PREFIX_SYMBOLS
  are defined, run util/make_prefix_headers.go to generate header
  files
- In various CMakeLists.txt files, add "global_target" that all
  targets depend on to give us a place to hook logic that must run
  before all other targets (in particular, the header file generation
  logic)
- Document this in BUILDING.md, including the fact that it is
  the caller's responsibility to provide the symbol list and keep it
  up to date
- Note that this scheme has not been tested on Windows, and likely
  does not work on it; Windows support will need to be added in a
  future commit

Change-Id: If66a7157f46b5b66230ef91e15826b910cf979a2
Reviewed-on: https://boringssl-review.googlesource.com/31364
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: David Benjamin <davidben@google.com>