open-mpi: add MPI_THREAD_MULTIPLE support.

Closes Homebrew/homebrew#16956.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/open-mpi.rb

@@ -15,6 +15,7 @@ class OpenMpi < Formula
 
   option 'disable-fortran', 'Do not build the Fortran bindings'
   option 'test', 'Verify the build with make check'
+  option 'enable-mpi-thread-multiple', 'Enable MPI_THREAD_MULTIPLE'
 
   def install
     args = %W[
@@ -28,6 +29,10 @@ class OpenMpi < Formula
       ENV.fortran
     end
 
+    if build.include? 'enable-mpi-thread-multiple'
+      args << '--enable-mpi-thread-multiple'
+    end
+
     system './configure', *args
     system 'make V=1 all'
     system 'make V=1 check' if build.include? 'test'