add alias targets with fmt namespace

For the consumer it should not matter if fmt has been added to the
project as subdirectory or via find_package. With the alias targets
the library can be always imported via fmt::fmt.

CMakeLists.txt

@@ -133,6 +133,7 @@ endif ()
 # Additionally define a header-only library when CMake is new enough.
 if (CMAKE_VERSION VERSION_GREATER 3.1.0 OR CMAKE_VERSION VERSION_EQUAL 3.1.0)
   add_library(fmt-header-only INTERFACE)
+  add_library(fmt::fmt-header-only ALIAS fmt-header-only)
 
   target_compile_definitions(fmt-header-only INTERFACE FMT_HEADER_ONLY=1)
 

test/add-subdirectory-test/CMakeLists.txt

@@ -6,10 +6,10 @@ add_subdirectory(../.. fmt)
 
 add_executable(library-test "main.cc")
 target_compile_options(library-test PUBLIC ${CPP14_FLAG})
-target_link_libraries(library-test fmt)
+target_link_libraries(library-test fmt::fmt)
 
-if (TARGET fmt-header-only)
+if (TARGET fmt::fmt-header-only)
   add_executable(header-only-test "main.cc")
   target_compile_options(header-only-test PUBLIC ${CPP14_FLAG})
-  target_link_libraries(header-only-test fmt-header-only)
+  target_link_libraries(header-only-test fmt::fmt-header-only)
 endif ()