corrected install options in cmake config

Author: kkiesling

cmake/OpenMCConfig.cmake.in

@@ -5,11 +5,11 @@ find_package(gsl-lite REQUIRED HINTS ${OpenMC_CMAKE_DIR}/../gsl-lite)
 find_package(pugixml REQUIRED HINTS ${OpenMC_CMAKE_DIR}/../pugixml)
 find_package(xtl REQUIRED HINTS ${OpenMC_CMAKE_DIR}/../xtl)
 find_package(xtensor REQUIRED HINTS ${OpenMC_CMAKE_DIR}/../xtensor)
-if(@dagmc@)
+if(@OPENMC_USE_DAGMC@)
   find_package(DAGMC REQUIRED HINTS @DAGMC_DIR@)
 endif()
 
-if(@libmesh@)
+if(@OPENMC_USE_LIBMESH@)
   include(FindPkgConfig)
   list(APPEND CMAKE_PREFIX_PATH @LIBMESH_PREFIX@)
   set(PKG_CONFIG_USE_CMAKE_PREFIX_PATH True)