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/builder.py

@@ -387,6 +387,7 @@ if __name__ == "__main__":
         inst_dir,
         defines,
         final_install_prefix=None,
+        extra_cmake_defines=None,
     ):
         super(CMakeBuilder, self).__init__(
             build_opts,
@@ -398,6 +399,8 @@ if __name__ == "__main__":
             final_install_prefix=final_install_prefix,
         )
         self.defines = defines or {}
+        if extra_cmake_defines:
+            self.defines.update(extra_cmake_defines)
 
     def _invalidate_cache(self):
         for name in [