old_contents: from spack import *
  
  class Mrnet(Package):
      """The MRNet Multi-Cast Reduction Network."""
      homepage = "http://paradyn.org/mrnet"
      url      = "ftp://ftp.cs.wisc.edu/paradyn/mrnet/mrnet_4.0.0.tar.gz"
  
      version('4.0.0', 'd00301c078cba57ef68613be32ceea2f')
      version('4.1.0', '5a248298b395b329e2371bf25366115c')
  
      parallel = False
  
      depends_on("boost")
  
      def install(self, spec, prefix):
          configure("--prefix=%s" %prefix, "--enable-shared")
  
          make()
          make("install")
  
  
new_contents: from spack import *
  
  class Mrnet(Package):
      """The MRNet Multi-Cast Reduction Network."""
      homepage = "http://paradyn.org/mrnet"
      url      = "ftp://ftp.cs.wisc.edu/paradyn/mrnet/mrnet_4.0.0.tar.gz"
  
      version('4.0.0', 'd00301c078cba57ef68613be32ceea2f')
      version('4.1.0', '5a248298b395b329e2371bf25366115c')
  
      variant('krelloptions', default=False, description="Also build the MRNet LW threadsafe libraries")
      parallel = False
  
      depends_on("boost")
  
      def install(self, spec, prefix):
          # Build the MRNet LW thread safe libraries when the krelloptions variant is present
          if '+krelloptions' in spec:
             configure("--prefix=%s" %prefix, "--enable-shared", "--enable-ltwt-threadsafe")
          else:
             configure("--prefix=%s" %prefix, "--enable-shared")
  
          make()
          make("install")
  
  
subject: Add krelloptions variant that is used to turn on a configuration option to build the thread safe lightweight libraries.