build: support system fmt dependency

cmake/VSAGHelpers.cmake

@@ -125,3 +125,31 @@ function (vsag_fail_missing_system_dep dep package hint)
              "Install package '${package}', expose target ${hint} before configuring VSAG, "
              "or set VSAG_USE_SYSTEM_${dep}=OFF to use the bundled copy.")
 endfunction ()
+
+# Return TRUE when ${target}'s declared include directories contain ${header}.
+function (vsag_target_has_header target header out_var)
+    set (_include_dirs "")
+    get_target_property (_target_includes ${target} INTERFACE_INCLUDE_DIRECTORIES)
+    if (_target_includes)
+        list (APPEND _include_dirs ${_target_includes})
+    endif ()
+
+    get_target_property (_target_system_includes ${target} INTERFACE_SYSTEM_INCLUDE_DIRECTORIES)
+    if (_target_system_includes)
+        list (APPEND _include_dirs ${_target_system_includes})
+    endif ()
+
+    set (_found FALSE)
+    foreach (_include_dir ${_include_dirs})
+        if (_include_dir MATCHES "^\\$<BUILD_INTERFACE:(.*)>$")
+            set (_include_dir "${CMAKE_MATCH_1}")
+        elseif (_include_dir MATCHES "^\\$<INSTALL_INTERFACE:.*>$")
+            continue ()
+        endif ()
+        if (_include_dir AND EXISTS "${_include_dir}/${header}")
+            set (_found TRUE)
+            break ()
+        endif ()
+    endforeach ()
+    set (${out_var} ${_found} PARENT_SCOPE)
+endfunction ()

cmake/VSAGOptions.cmake

@@ -64,6 +64,10 @@ set (VSAG_USE_SYSTEM_OPENBLAS "" CACHE STRING
      "Override VSAG_USE_SYSTEM_DEPS for OpenBLAS: AUTO, ON, OFF, or empty to inherit.")
 set_property (CACHE VSAG_USE_SYSTEM_OPENBLAS PROPERTY STRINGS "" AUTO ON OFF)
 
+set (VSAG_USE_SYSTEM_FMT "" CACHE STRING
+     "Override VSAG_USE_SYSTEM_DEPS for fmt: AUTO, ON, OFF, or empty to inherit.")
+set_property (CACHE VSAG_USE_SYSTEM_FMT PROPERTY STRINGS "" AUTO ON OFF)
+
 set (_default_aclocal_path "")
 if (DEFINED ENV{ACLOCAL_PATH} AND NOT "$ENV{ACLOCAL_PATH}" STREQUAL "")
     set (_default_aclocal_path "$ENV{ACLOCAL_PATH}")

extern/fmt/fmt.cmake

@@ -5,6 +5,67 @@ include (FetchContent)
 # ref: https://github.com/fmtlib/fmt/issues/2708
 set (FMT_SYSTEM_HEADERS ON)
 
+vsag_get_system_dep_policy (FMT _fmt_policy)
+set (FMT_FOUND FALSE)
+
+if (NOT _fmt_policy STREQUAL "OFF")
+    if (TARGET fmt::fmt)
+        vsag_target_has_header (fmt::fmt fmt/format.h _fmt_has_header)
+        if (_fmt_has_header)
+            set (FMT_FOUND TRUE)
+            message (STATUS "Using pre-existing fmt::fmt target")
+        else ()
+            message (STATUS "Ignoring pre-existing fmt::fmt target without fmt/format.h")
+        endif ()
+    endif ()
+
+    if (NOT FMT_FOUND)
+        set (_fmt_include_hints "")
+        if (DEFINED fmt_DIR AND NOT "${fmt_DIR}" STREQUAL "")
+            get_filename_component (_fmt_prefix "${fmt_DIR}/../../.." ABSOLUTE)
+            list (APPEND _fmt_include_hints "${_fmt_prefix}" "${_fmt_prefix}/include")
+        endif ()
+
+        unset (_fmt_include_dir CACHE)
+        unset (_vsag_fmt_include_dir CACHE)
+        find_path (_vsag_fmt_include_dir
+            NAMES fmt/format.h
+            HINTS ${_fmt_include_hints}
+            PATH_SUFFIXES include)
+        set (_fmt_include_dir "${_vsag_fmt_include_dir}")
+        unset (_vsag_fmt_include_dir CACHE)
+
+        if (_fmt_include_dir)
+            find_package (fmt CONFIG QUIET)
+            if (TARGET fmt::fmt)
+                vsag_target_has_header (fmt::fmt fmt/format.h _fmt_has_header)
+                if (_fmt_has_header)
+                    set (FMT_FOUND TRUE)
+                    message (STATUS "Found fmt via find_package(fmt CONFIG)")
+                else ()
+                    message (STATUS "Ignoring fmt package without fmt/format.h")
+                endif ()
+            else ()
+                message (STATUS "Ignoring fmt package without fmt::fmt target")
+            endif ()
+        else ()
+            message (STATUS "fmt/format.h was not found in system include paths")
+        endif ()
+    endif ()
+
+    if (NOT FMT_FOUND AND _fmt_policy STREQUAL "ON")
+        vsag_fail_missing_system_dep (FMT fmt fmt::fmt)
+    endif ()
+endif ()
+
+if (FMT_FOUND)
+    if (NOT TARGET fmt::fmt-header-only)
+        add_library (fmt::fmt-header-only INTERFACE IMPORTED GLOBAL)
+        target_link_libraries (fmt::fmt-header-only INTERFACE fmt::fmt)
+    endif ()
+    return ()
+endif ()
+
 set (fmt_urls
     https://github.com/fmtlib/fmt/archive/refs/tags/10.2.1.tar.gz
     # this url is maintained by the vsag project, if it's broken, please try