Bug 1747145 - Only apply stlport flags to configure compiler executions for target. r=firefox-build-system-reviewers,andi

(as opposed to host or wasm)

Differential Revision: https://phabricator.services.mozilla.com/D167112

build/moz.configure/compilers-util.configure

@@ -109,8 +109,8 @@ def compiler_class(compiler, host_or_target):
                 flags = list(flags or [])
                 if is_target:
                     flags += extra_flags or []
-                if compiler.language == "C++":
-                    flags += stlport_flags or []
+                    if compiler.language == "C++":
+                        flags += stlport_flags or []
                 header = header or ""
                 if isinstance(header, (list, tuple)):
                     header = "\n".join(header)