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/load.py

@@ -275,7 +275,14 @@ class ManifestLoader(object):
         env["os"] = self.build_opts.host_type.ostype
         env["distro"] = self.build_opts.host_type.distro
         env["distro_vers"] = self.build_opts.host_type.distrovers
-        for name in ["CXXFLAGS", "CPPFLAGS", "LDFLAGS", "CXX", "CC"]:
+        for name in [
+            "CXXFLAGS",
+            "CPPFLAGS",
+            "LDFLAGS",
+            "CXX",
+            "CC",
+            "GETDEPS_CMAKE_DEFINES",
+        ]:
             env[name] = os.environ.get(name)
         for tool in ["cc", "c++", "gcc", "g++", "clang", "clang++"]:
             env["tool-%s" % tool] = path_search(os.environ, tool)