Fix cross compiling for android on macOS

The way rules_foreign_cc builds cross compilation targets isn't exactly
what cmake expects. Because of this when targeting android on macOS,
some macOS specific linker flags are added to cmake's test compile
invocations, leading to failures. This works around this issue by
setting some more variables to trick cmake into only using the expected
settings.

foreign_cc/cmake.bzl

@@ -148,6 +148,7 @@ load("//foreign_cc/private:transitions.bzl", "foreign_cc_rule_variant")
 load(
     "//foreign_cc/private/framework:platform.bzl",
     "os_name",
+    "target_os_name",
 )
 load(
     "//toolchains/native_tools:tool_access.bzl",
@@ -251,6 +252,7 @@ def _create_configure_script(configureParameters):
 
     configure_script = create_cmake_script(
         workspace_name = ctx.workspace_name,
+        target_os = target_os_name(ctx),
         generator = attrs.generator,
         cmake_path = attrs.cmake_path,
         tools = tools,

foreign_cc/private/cmake_script.bzl

@@ -19,6 +19,7 @@ def _escape_dquote_bash_crosstool(text):
 
 def create_cmake_script(
         workspace_name,
+        target_os,
         generator,
         cmake_path,
         tools,
@@ -37,6 +38,7 @@ def create_cmake_script(
 
     Args:
         workspace_name: current workspace name
+        target_os: The target OS for the build
         generator: The generator target for cmake to use
         cmake_path: The path to the cmake executable
         tools: cc_toolchain tools (CxxToolsInfo)
@@ -91,6 +93,14 @@ def create_cmake_script(
     if not params.cache.get("CMAKE_RANLIB"):
         params.cache.update({"CMAKE_RANLIB": ""})
 
+    # Avoid cmake passing wrong linker flags when targeting android on macOS
+    # https://github.com/bazelbuild/rules_foreign_cc/issues/289
+    if target_os == "android":
+        params.cache.update({
+            "ANDROID": "YES",
+            "CMAKE_SYSTEM_NAME": "Linux",
+        })
+
     set_env_vars = [
         "export {}=\"{}\"".format(key, _escape_dquote_bash(params.env[key]))
         for key in params.env

foreign_cc/private/framework.bzl

@@ -218,6 +218,7 @@ CC_EXTERNAL_RULE_ATTRIBUTES = {
         cfg = "exec",
         default = Label("@rules_foreign_cc//foreign_cc/private/framework:platform_info"),
     ),
+    "_android_constraint": attr.label(default = Label("@platforms//os:android")),
 }
 
 # A list of common fragments required by rules using this framework

foreign_cc/private/framework/platform.bzl

@@ -65,3 +65,22 @@ def os_name(ctx):
         return "unknown"
 
     return platform_info[ForeignCcPlatformInfo].os
+
+def target_os_name(ctx):
+    """A helper function for getting the target operating system name based on the constraints
+
+    Args:
+        ctx (ctx): The current rule's context object
+
+    Returns:
+        str: The string of the current platform
+    """
+
+    android_constraint = ctx.attr._android_constraint[platform_common.ConstraintValueInfo]
+    operating_systems = ["android"]
+    for os in operating_systems:
+        constraint = getattr(ctx.attr, "_{}_constraint".format(os))
+        if constraint and ctx.target_platform_has_constraint(constraint[platform_common.ConstraintValueInfo]):
+            return os
+
+    return "unknown"