make fmt optional for the llama target

Fixes #382.

CMakeLists.txt

@@ -3,11 +3,16 @@ project (llama CXX)
 
 # llama
 find_package(Boost 1.70.0 REQUIRED)
-find_package(fmt CONFIG REQUIRED)
+find_package(fmt CONFIG QUIET)
 add_library(${PROJECT_NAME} INTERFACE)
 target_include_directories(${PROJECT_NAME} INTERFACE $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include> $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
 target_compile_features(${PROJECT_NAME} INTERFACE cxx_std_17)
-target_link_libraries(${PROJECT_NAME} INTERFACE Boost::headers fmt::fmt)
+target_link_libraries(${PROJECT_NAME} INTERFACE Boost::headers)
+if (fmt_FOUND)
+	target_link_libraries(${PROJECT_NAME} INTERFACE fmt::fmt)
+else()
+	message("The fmt library was not found. You cannot use llama/DumpMapping.hpp")
+endif()
 
 # llama::llama to make subdirectory projects work
 add_library(${PROJECT_NAME}::${PROJECT_NAME} ALIAS ${PROJECT_NAME})

include/llama/DumpMapping.hpp

@@ -2,6 +2,10 @@
 
 #pragma once
 
+#if !__has_include(<fmt/format.h>)
+#    error DumpMapping.hpp requires the fmt library
+#endif
+
 #include "ArrayDimsIndexRange.hpp"
 #include "Core.hpp"