update python to 3.8

Summary:
Update to a newer python that builds on macOS Big Sur, make eden_scm depend on it and use it from PATH

python 3.8 requires libffi which is reference via its pkg-config setup,  however python's pkg-config libffi detection is broken (https://bugs.python.org/issue34823) with the documented workaround requiring an environment variable to be passed into its ./configure step, which is why this change also adds a feature to AutoconfBuilder

With the updated python in place I was able to remove disable_env_override_pkgconfig = 1
disable_env_override_path = 1 from the eden_scm config so that it actually uses the pkg-configs, PATHs and other settings from its dependencies. This should make future python and other dependency upgrades much simpler.

Reviewed By: HarveyHunt

Differential Revision: D32231261

fbshipit-source-id: a2b6addbe22f38e3d71618c802d2c6f836fdd86c

build/fbcode_builder/getdeps/manifest.py

@@ -69,8 +69,6 @@ SCHEMA = {
             "builder": REQUIRED,
             "subdir": OPTIONAL,
             "build_in_src_dir": OPTIONAL,
-            "disable_env_override_pkgconfig": OPTIONAL,
-            "disable_env_override_path": OPTIONAL,
         },
     },
     "msbuild": {"optional_section": True, "fields": {"project": REQUIRED}},
@@ -84,6 +82,7 @@ SCHEMA = {
     },
     "cmake.defines": {"optional_section": True},
     "autoconf.args": {"optional_section": True},
+    "autoconf.envcmd.LDFLAGS": {"optional_section": True},
     "rpms": {"optional_section": True},
     "debs": {"optional_section": True},
     "preinstalled.env": {"optional_section": True},
@@ -101,6 +100,7 @@ SCHEMA = {
 # using the expression syntax to enable/disable sections
 ALLOWED_EXPR_SECTIONS = [
     "autoconf.args",
+    "autoconf.envcmd.LDFLAGS",
     "build",
     "cmake.defines",
     "dependencies",
@@ -469,8 +469,19 @@ class ManifestParser(object):
 
         if builder == "autoconf":
             args = self.get_section_as_args("autoconf.args", ctx)
+            conf_env_args = {}
+            ldflags_cmd = self.get_section_as_args("autoconf.envcmd.LDFLAGS", ctx)
+            if ldflags_cmd:
+                conf_env_args["LDFLAGS"] = ldflags_cmd
             return AutoconfBuilder(
-                build_options, ctx, self, src_dir, build_dir, inst_dir, args
+                build_options,
+                ctx,
+                self,
+                src_dir,
+                build_dir,
+                inst_dir,
+                args,
+                conf_env_args,
             )
 
         if builder == "boost":