Enable passing vars to configure_file with flags during build [BUILD-391]

configure_file.bzl

@@ -1,8 +1,17 @@
+load("@bazel_skylib//rules:common_settings.bzl", "string_flag")
+
 CMAKE_FALSE_CONSTANTS = ["0", "OFF", "NO", "FALSE", "N", "IGNORE", "NOTFOUND"]
 
 def _configure_file_impl(ctx):
     vars = {}
-    for (key, val) in ctx.attr.vars.items():
+    for var in ctx.attr.vars:
+        key_val = var.split("=")
+        if len(key_val) != 2:
+            # skip if var is not in the right format: <key>=<value>
+            continue
+        key = key_val[0]
+        val = key_val[1]
+
         cmake_define = "#cmakedefine {}".format(key)
         define = "// #undef {}".format(key) if val in CMAKE_FALSE_CONSTANTS else "#define {}".format(key)
 
@@ -21,11 +30,31 @@ def _configure_file_impl(ctx):
 configure_file = rule(
     implementation = _configure_file_impl,
     attrs = {
-        "vars": attr.string_dict(),
+        "vars": attr.string_list(),
         "out": attr.string(),
         "template": attr.label(
             allow_single_file = [".in"],
             mandatory = True,
         ),
     },
 )
+
+def config_var(var, config):
+    return select({
+        config: [var + "=ON"],
+        "//conditions:default": [var + "=OFF"],
+    })
+
+def var(var, value):
+    return [var + "=" + value]
+
+def config_flag(config, flag):
+    string_flag(
+        name = flag,
+        build_setting_default = "",
+    )
+
+    native.config_setting(
+        name = config,
+        flag_values = {":" + flag: "ON"},
+    )