open-mpi: always build with C++11

Closes #21763.

Signed-off-by: ilovezfs <ilovezfs@icloud.com>

Formula/open-mpi.rb

@@ -3,6 +3,7 @@ class OpenMpi < Formula
   homepage "https://www.open-mpi.org/"
   url "https://www.open-mpi.org/software/ompi/v3.0/downloads/openmpi-3.0.0.tar.bz2"
   sha256 "f699bff21db0125d8cccfe79518b77641cd83628725a1e1ed3e45633496a82d7"
+  revision 1
 
   bottle do
     sha256 "f1a885c11086fa6a2ead5ec91d88a9bf6234f8ad4ccc2730a9b4798c70c8d1b5" => :high_sierra
@@ -19,7 +20,6 @@ class OpenMpi < Formula
 
   option "with-mpi-thread-multiple", "Enable MPI_THREAD_MULTIPLE"
   option "with-cxx-bindings", "Enable C++ MPI bindings (deprecated as of MPI-3.0)"
-  option :cxx11
 
   deprecated_option "disable-fortran" => "without-fortran"
   deprecated_option "enable-mpi-thread-multiple" => "with-mpi-thread-multiple"
@@ -31,11 +31,13 @@ class OpenMpi < Formula
   conflicts_with "mpich", :because => "both install mpi__ compiler wrappers"
   conflicts_with "lcdf-typetools", :because => "both install same set of binaries."
 
+  needs :cxx11
+
   def install
     # otherwise libmpi_usempi_ignore_tkr gets built as a static library
     ENV["MACOSX_DEPLOYMENT_TARGET"] = MacOS.version
 
-    ENV.cxx11 if build.cxx11?
+    ENV.cxx11
 
     args = %W[
       --prefix=#{prefix}