Prepend user-specified PATH to existing PATH (#733)

If a user specifies a PATH value as part of an `env` attribute, the
value will be prepended to the existing PATH.

An example requirement for this change is that the MSVC build of
OpenSSL requires that the Netwide Assembler (NASM) must be on the
PATH.

foreign_cc/private/framework.bzl

@@ -103,7 +103,8 @@ CC_EXTERNAL_RULE_ATTRIBUTES = {
             "`$(execpath)` macros may be used to point at files which are listed as `data`, `deps`, or `build_data`, " +
             "but unlike with other rules, these will be replaced with absolute paths to those files, " +
             "because the build does not run in the exec root. " +
-            "No other macros are supported."
+            "No other macros are supported." +
+            "Variables containing `PATH` (e.g. `PATH`, `LD_LIBRARY_PATH`, `CPATH`) entries will be prepended to the existing variable."
         ),
     ),
     "lib_name": attr.string(
@@ -299,7 +300,14 @@ def get_env_prelude(ctx, lib_name, data_dependencies, target_root):
     env.update(cc_env)
 
     # Add all user defined variables
-    env.update(expand_locations(ctx, ctx.attr.env, data_dependencies))
+    user_vars = expand_locations(ctx, ctx.attr.env, data_dependencies)
+    env.update(user_vars)
+
+    # If user has defined a PATH variable (e.g. PATH, LD_LIBRARY_PATH, CPATH) prepend it to the existing variable
+    for user_var in user_vars:
+        if "PATH" in user_var and cc_env.get(user_var):
+            env.update({user_var: user_vars.get(user_var) + ":" + cc_env.get(user_var)})
+
     env_snippet.extend(["export {}=\"{}\"".format(key, _escape_dquote(val)) for key, val in env.items()])
 
     return env_snippet