common: add subproc.h wrapper, disabled on android/ios (#26102)

* add common/subproc.h|cpp

* add compile flag LLAMA_SUBPROCESS

* disabled by default on android and ios

* test-jinja: use common subproc

* mtmd: disable video if subproc is not set

* disable subproc on wasm

* make is_created atomic

* migrate server-mcp

CMakeLists.txt

@@ -84,6 +84,14 @@ else()
     set(LLAMA_TOOLS_INSTALL_DEFAULT ${LLAMA_STANDALONE})
 endif()
 
+# subprocess spawning isn't a supported/sandbox-friendly operation on mobile OSes or in WASM
+if (CMAKE_SYSTEM_NAME STREQUAL "iOS" OR CMAKE_SYSTEM_NAME STREQUAL "Android" OR ANDROID
+        OR CMAKE_SYSTEM_NAME STREQUAL "Emscripten" OR EMSCRIPTEN)
+    set(LLAMA_SUBPROCESS_DEFAULT OFF)
+else()
+    set(LLAMA_SUBPROCESS_DEFAULT ON)
+endif()
+
 #
 # option list
 #
@@ -117,6 +125,7 @@ option(LLAMA_TESTS_INSTALL "llama: install tests" ON)
 
 # 3rd party libs
 option(LLAMA_OPENSSL    "llama: use openssl to support HTTPS" ON)
+option(LLAMA_SUBPROCESS "llama-common: use subprocess, required by server tools and server router mode" ${LLAMA_SUBPROCESS_DEFAULT})
 option(LLAMA_LLGUIDANCE "llama-common: include LLGuidance library for structured output in common utils" OFF)