#2216: Add CMake option to enable using external fmt

cmake/configure_options.cmake

@@ -151,6 +151,13 @@ define_option(vt_build_tests "tests" "Build VT tests" ON empty_feature)
 define_option(vt_build_tools "tools" "Build VT tools" ON empty_feature)
 define_option(vt_build_examples "examples" "Build VT examples" ON empty_feature)
 
+option(vt_external_fmt "Build VT with external fmt" OFF)
+if(${vt_external_fmt})
+    message(STATUS "Building vt with external fmt library")
+else()
+    message(STATUS "Building vt with included fmt library")
+endif()
+
 list(POP_BACK CMAKE_MESSAGE_INDENT)
 
 ##########################################################################################################

cmake/load_bundled_libraries.cmake

@@ -20,10 +20,21 @@ endif()
 # CLI11 always included in the build
 add_subdirectory(${PROJECT_LIB_DIR}/CLI)
 
-# fmt always included in the build
-set(FMT_LIBRARY fmt)
-add_subdirectory(${PROJECT_LIB_DIR}/fmt)
-set_darma_compiler_flags(${FMT_LIBRARY})
+# use included fmt or external one
+if(${vt_external_fmt})
+  # user should provide 'fmt_DIR' to CMake (unless fmt is installed in system libs)
+  if(${fmt_DIR})
+    message(STATUS "vt_external_fmt = ON. Using fmt located at ${fmt_DIR}")
+  else()
+    message(STATUS "vt_external_fmt = ON but ${fmt_DIR} is not provided.")
+  endif()
+  find_package(fmt REQUIRED)
+else()
+  set(FMT_LIBRARY fmt)
+  add_subdirectory(${PROJECT_LIB_DIR}/fmt)
+  set_darma_compiler_flags(${FMT_LIBRARY})
+endif()
+
 
 # EngFormat-Cpp always included in the build
 set(ENG_FORMAT_LIBRARY EngFormat-Cpp)