See PR pending https://github.com/libsidplayfp/sidplayfp/pull/135.patch
add --with-system-fmt option (disabled by default)
move bundled fmt into fmt_contrib to avoid conflicts
--- a/Makefile.am
+++ b/Makefile.am
@@ -26,23 +26,29 @@ src/stilview
 
 #=========================================================
 #fmt
+if USE_SYSTEM_FMT
+AM_CPPFLAGS += $(FMT_CFLAGS)
+fmt_SOURCES =
+else
+AM_CPPFLAGS += -I $(top_srcdir)/libs/fmt_internal
 fmt_SOURCES = \
-libs/fmt/args.h \
-libs/fmt/base.h \
-libs/fmt/chrono.h \
-libs/fmt/color.h \
-libs/fmt/compile.h \
-libs/fmt/core.h \
-libs/fmt/format-inl.h \
-libs/fmt/format.cc \
-libs/fmt/format.h \
-libs/fmt/os.cc \
-libs/fmt/os.h \
-libs/fmt/ostream.h \
-libs/fmt/printf.h \
-libs/fmt/ranges.h \
-libs/fmt/std.h \
-libs/fmt/xchar.h
+libs/fmt_internal/fmt/args.h \
+libs/fmt_internal/fmt/base.h \
+libs/fmt_internal/fmt/chrono.h \
+libs/fmt_internal/fmt/color.h \
+libs/fmt_internal/fmt/compile.h \
+libs/fmt_internal/fmt/core.h \
+libs/fmt_internal/fmt/format-inl.h \
+libs/fmt_internal/fmt/format.cc \
+libs/fmt_internal/fmt/format.h \
+libs/fmt_internal/fmt/os.cc \
+libs/fmt_internal/fmt/os.h \
+libs/fmt_internal/fmt/ostream.h \
+libs/fmt_internal/fmt/printf.h \
+libs/fmt_internal/fmt/ranges.h \
+libs/fmt_internal/fmt/std.h \
+libs/fmt_internal/fmt/xchar.h
+endif
 
 #=========================================================
 # sidplayfp
@@ -95,7 +101,8 @@ $(PTHREAD_CFLAGS)
 src_sidplayfp_LDADD = \
 $(SIDPLAYFP_LIBS) \
 $(W32_LIBS) \
-$(PTHREAD_LIBS)
+$(PTHREAD_LIBS) \
+$(FMT_LIBS)
 
 #=========================================================
 # stilview
@@ -107,7 +114,8 @@ src/codeConvert.h \
 src/stilview.cpp
 
 src_stilview_LDADD = \
-$(STILVIEW_LIBS)
+$(STILVIEW_LIBS) \
+$(FMT_LIBS)
 
 #=========================================================
 # docs
--- a/configure.ac
+++ b/configure.ac
@@ -94,6 +94,20 @@ AS_IF([test "x$enable_debug" = "xno"],
   [AC_MSG_RESULT([yes])],
 )
 
+AC_ARG_WITH([system-fmt], AS_HELP_STRING([--with-system-fmt], [use system fmt library instead of bundled version]),
+    [with_system_fmt=$withval],
+    [with_system_fmt=no]
+)
+
+AS_IF([test "x$with_system_fmt" = "xyes"], [
+    PKG_CHECK_MODULES([FMT], [fmt >= 11.0.0])
+    AC_DEFINE([USE_SYSTEM_FMT], [1], [Use system fmt])
+], [
+    AC_MSG_NOTICE([Using bundled fmt library])
+])
+
+AM_CONDITIONAL([USE_SYSTEM_FMT], [test "x$with_system_fmt" = "xyes"])
+
 AC_SUBST([debug_flags])
 
 AC_CONFIG_FILES([
--- a/src/IniConfig.cpp
+++ b/src/IniConfig.cpp
@@ -25,7 +25,7 @@
 
 #include "IniConfig.h"
 
-#include "fmt/format.h"
+#include <fmt/format.h>
 
 #include <string>
 
--- a/src/args.cpp
+++ b/src/args.cpp
@@ -23,7 +23,8 @@
 
 #include "dataParser.h"
 #include "utils.h"
-#include "fmt/format.h"
+
+#include <fmt/format.h>
 
 #include <iostream>
 
--- a/src/menu.cpp
+++ b/src/menu.cpp
@@ -22,9 +22,10 @@
 #include "player.h"
 
 #include "codeConvert.h"
-#include "fmt/format.h"
+
+#include <fmt/format.h>
 #if defined(_WIN32) && defined(UNICODE)
-#  include "fmt/xchar.h"
+#  include <fmt/xchar.h>
 #endif
 
 #include <cctype>
--- a/src/player.cpp
+++ b/src/player.cpp
@@ -21,9 +21,10 @@
 
 #include "player.h"
 
-#include "fmt/format.h"
 #include "filesystem/filesystem.hpp"
 
+#include <fmt/format.h>
+
 #include <cstdlib>
 #include <cmath>
 #include <cstring>
--- a/src/siddefines.h
+++ b/src/siddefines.h
@@ -22,7 +22,7 @@
 #ifndef SIDDEFINES_H
 #define SIDDEFINES_H
 
-#include "fmt/color.h"
+#include <fmt/color.h>
 
 using color_t = fmt::terminal_color;
 
--- a/src/stilview.cpp
+++ b/src/stilview.cpp
@@ -23,7 +23,7 @@
 // STILView - command line version
 //
 
-#include "fmt/format.h"
+#include <fmt/format.h>
 
 #include <fstream>
 #include <iostream>