[build-script] Add infrastructure for passing host-specific variables.

 - This isn't yet used, but we need an easy way for `build-script` to
   pass host-specific variables down to `build-script-impl`.

 - I don't think it is worth complicating the main argument parsing
   logic with a syntax for passing associative arrays on the command
   line, so this just passes them via environment variables.

 - Part of SR-237.

Author: ddunbar

utils/build-script

@@ -50,7 +50,7 @@ from swift_build_support.cmake import CMake  # noqa (E402)
 import swift_build_support.workspace    # noqa (E402)
 
 
-def call_without_sleeping(command, dry_run=False):
+def call_without_sleeping(command, env=None, dry_run=False):
     """
     Execute a command during which system sleep is disabled.
 
@@ -62,7 +62,7 @@ def call_without_sleeping(command, dry_run=False):
         # Don't mutate the caller's copy of the arguments.
         command = ["caffeinate"] + list(command)
 
-    shell.call(command, dry_run=dry_run, echo=False)
+    shell.call(command, env=env, dry_run=dry_run)
 
 
 class BuildScriptInvocation(object):
@@ -322,10 +322,10 @@ class BuildScriptInvocation(object):
         self.toolchain.ninja = ninja_build.ninja_bin_path
 
     def convert_to_impl_arguments(self):
-        """convert_to_impl_arguments() -> args
+        """convert_to_impl_arguments() -> (env, args)
 
-        Convert the invocation to a list of arguments suitable for invoking
-        `build-script-impl`.
+        Convert the invocation to an environment and list of arguments suitable
+        for invoking `build-script-impl`.
         """
 
         # Create local shadows, for convenience.
@@ -540,7 +540,26 @@ class BuildScriptInvocation(object):
         if args.dry_run:
             impl_args += ["--dry-run"]
 
-        return impl_args
+        # Compute the set of host-specific variables, which we pass through to
+        # the build script via environment variables.
+        host_specific_variables = self.compute_host_specific_variables()
+        impl_env = {}
+        for (host_target, options) in host_specific_variables.items():
+            for (name, value) in options.items():
+                # We mangle into an environment variable we can easily evaluate
+                # from the `build-script-impl`.
+                impl_env["HOST_VARIABLE_{}__{}".format(
+                    host_target.replace("-", "_"), name)] = value
+
+        return (impl_env, impl_args)
+
+    def compute_host_specific_variables(self):
+        """compute_host_specific_variables(args) -> dict
+
+        Compute the host-specific options, organized as a dictionary keyed by
+        host of options.
+        """
+        return {}
 
 
 # Main entry point for the preset mode.
@@ -1551,10 +1570,12 @@ details of the setups of other systems or automated environments.""")
         invocation.build_ninja()
 
     # Convert to a build-script-impl invocation.
-    build_script_impl_args = invocation.convert_to_impl_arguments()
+    (build_script_impl_env,build_script_impl_args) = \
+        invocation.convert_to_impl_arguments()
 
     # Execute the underlying build script implementation.
-    call_without_sleeping([build_script_impl] + build_script_impl_args)
+    call_without_sleeping([build_script_impl] + build_script_impl_args,
+                          env=build_script_impl_env)
 
     if args.symbols_package:
         print('--- Creating symbols package ---')

utils/build-script-impl

@@ -1138,6 +1138,19 @@ if [[ ! "${SKIP_BUILD_SWIFTPM}" ]] ; then
      PRODUCTS=("${PRODUCTS[@]}" swiftpm)
 fi
 
+# get_host_specific_variable(host, name)
+#
+# Get the value of a host-specific variable expected to have been passed by the
+# `build-script`.
+#
+# This is a total hack, and is part of the SR-237 migration.
+function get_host_specific_variable() {
+    local host="$1"
+    local name="$2"
+    local envvar_name="HOST_VARIABLE_${host//-/_}__${name}"
+    echo "${!envvar_name}"
+}
+
 function calculate_targets_for_host() {
     local host=$1