Added option for compiler in Makefile and made compiler flags dependent on which compiler one is using.
Added option for compiler in Makefile and made compiler flags dependent on which compiler one is using.

nhorelik/openmc,amandalund/openmc,smharper/openmc,samuelshaner/openmc,bhermanmit/cdash,nhorelik/openmc,wbinventor/openmc,johnnyliu27/openmc,bhermanmit/openmc,walshjon/openmc,mit-crpg/openmc,mit-crpg/openmc,johnnyliu27/openmc,walshjon/openmc,samuelshaner/openmc,amandalund/openmc,mit-crpg/openmc,liangjg/openmc,shikhar413/openmc,mjlong/openmc,sxds/opemmc,liangjg/openmc,liangjg/openmc,smharper/openmc,keadyk/openmc_mg_prepush,keadyk/openmc_mg_prepush,johnnyliu27/openmc,wbinventor/openmc,walshjon/openmc,liangjg/openmc,mit-crpg/openmc,lilulu/openmc,amandalund/openmc,mjlong/openmc,lilulu/openmc,kellyrowland/openmc,wbinventor/openmc,shenqicang/openmc,keadyk/openmc_mg_prepush,walshjon/openmc,shikhar413/openmc,bhermanmit/openmc,wbinventor/openmc,samuelshaner/openmc,samuelshaner/openmc,shenqicang/openmc,shikhar413/openmc,amandalund/openmc,paulromano/openmc,paulromano/openmc,smharper/openmc,shikhar413/openmc,paulromano/openmc,smharper/openmc,sxds/opemmc,johnnyliu27/openmc,paulromano/openmc,kellyrowland/openmc,lilulu/openmc