Add support for extra_cmake_defines

Summary:
`extra_cmake_defines` are extra flags that are passed to cmake when
compiling each one of the dependencies. For instance:
```
$ ./opensource/fbcode_builder/getdeps.py  build f4d --extra-cmake-define='{"CMAKE_CXX_FLAGS": "-mavx2 -mfma -mavx -mf16c -march=native"}'
```
It takes a json map as input, which can take a list of defines (key value
pairs).

Reviewed By: wez

Differential Revision: D25855781

fbshipit-source-id: 7f4fef2c66f4d12f23c8d7086d6a4f24fcc01ff7

build/fbcode_builder/getdeps/buildopts.py

@@ -436,6 +436,10 @@ def setup_build_options(args, host_type=None):
     if not is_windows():
         scratch_dir = os.path.realpath(scratch_dir)
 
+    # Save any extra cmake defines passed by the user in an env variable, so it
+    # can be used while hashing this build.
+    os.environ["GETDEPS_CMAKE_DEFINES"] = getattr(args, "extra_cmake_defines", "") or ""
+
     host_type = _check_host_type(args, host_type)
 
     return BuildOptions(