use pkg-config to find libfmt

This allows using the system libfmt on OpenBSD and NetBSD.
Unfortunately, the system libfmt is too old on OpenBSD at the moment,
and Dolphin doesn't currently compile on NetBSD for other reasons.

CMakeLists.txt

@@ -628,7 +628,9 @@ if (_M_X86_64)
 endif()
 add_subdirectory(Externals/cpp-optparse)
 
-dolphin_find_optional_system_library(fmt Externals/fmt 10.1)
+dolphin_find_optional_system_library_pkgconfig(FMT
+  fmt>=10.1 fmt::fmt Externals/fmt
+)
 
 add_subdirectory(Externals/imgui)
 add_subdirectory(Externals/implot)