whisper : add option to use system-installed GGML (#2887)

CMakeLists.txt

@@ -62,7 +62,8 @@ option(WHISPER_ALL_WARNINGS           "whisper: enable all compiler warnings"
 option(WHISPER_ALL_WARNINGS_3RD_PARTY "whisper: enable all compiler warnings in 3rd party libs" OFF)
 
 # build
-option(WHISPER_FATAL_WARNINGS "whisper: enable -Werror flag" OFF)
+option(WHISPER_FATAL_WARNINGS  "whisper: enable -Werror flag"               OFF)
+option(WHISPER_USE_SYSTEM_GGML "whisper: use system-installed GGML library" OFF)
 
 # sanitizers
 option(WHISPER_SANITIZE_THREAD    "whisper: enable thread sanitizer"    OFF)
@@ -121,7 +122,15 @@ whisper_option_depr(WARNING     WHISPER_SYCL_F16            GGML_SYCL_F16)
 #
 
 if (NOT TARGET ggml)
-    add_subdirectory(ggml)
+    if (WHISPER_USE_SYSTEM_GGML)
+        find_package(ggml REQUIRED)
+        if (NOT ggml_FOUND)
+            message(FATAL_ERROR "System-installed GGML library not found.")
+        endif()
+        add_library(ggml ALIAS ggml::ggml)
+    else()
+        add_subdirectory(ggml)
+    endif()
     # ... otherwise assume ggml is added by a parent CMakeLists.txt
 endif()
 add_subdirectory(src)