Don’t override environment variables when spawning

Author: mxcl

Sources/POSIX/system.swift

@@ -45,14 +45,16 @@ public func system() {}
 
 /// Convenience wrapper for posix_spawn.
 func posix_spawnp(path: String, args: [String], environment: [String: String] = [:], fileActions: posix_spawn_file_actions_t? = nil) throws -> pid_t {
-    var environment = environment
     let argv = args.map{ $0.withCString(strdup) }
     defer { for arg in argv { free(arg) } }
 
+    var environment = environment
     for key in ["PATH", "SDKROOT", "TOOLCHAINS", "HOME", "SWIFT_EXEC",
                 // FIXME these
                 "SPM_INSTALL_PATH", "SWIFT_BUILD_TOOL"] {
-        environment[key] = POSIX.getenv(key)
+        if environment[key] == nil {
+            environment[key] = POSIX.getenv(key)
+        }
     }
 
     let env = environment.map{ "\($0.0)=\($0.1)".withCString(strdup) }