build: UNITS_USE_LIBFMT support improved

src/core-fmt/CMakeLists.txt

@@ -22,14 +22,13 @@
 
 cmake_minimum_required(VERSION 3.15)
 
-option(UNITS_USE_LIBFMT "Enables usage of libfmt instead of the one from 'std'" OFF)
+option(UNITS_USE_LIBFMT "Enables usage of libfmt instead of the one from 'std'" ON)
 message(STATUS "UNITS_USE_LIBFMT: ${UNITS_USE_LIBFMT}")
 
-# find dependencies
-find_package(fmt CONFIG REQUIRED)
-
 add_units_module(core-fmt mp-units::core)
+target_compile_definitions(mp-units-core-fmt INTERFACE UNITS_USE_LIBFMT=$<BOOL:${UNITS_USE_LIBFMT}>)
+
 if(UNITS_USE_LIBFMT)
+  find_package(fmt CONFIG REQUIRED)
   target_link_libraries(mp-units-core-fmt INTERFACE fmt::fmt)
-  target_compile_definitions(mp-units-core-fmt INTERFACE UNITS_USE_LIBFMT)
 endif()

src/core-fmt/include/units/bits/fmt_hacks.h

@@ -29,6 +29,10 @@
 
 #include <units/bits/external/hacks.h>
 
+#ifndef UNITS_USE_LIBFMT
+#define UNITS_USE_LIBFMT 1
+#endif
+
 #if UNITS_USE_LIBFMT
 
 UNITS_DIAGNOSTIC_PUSH