linux build fix

Author: TomKaltz

src/CMakeModules/Bootstrap_Linux.cmake

@@ -39,12 +39,15 @@ find_package(OpenAL REQUIRED)
 find_package(SFML 2 COMPONENTS graphics window REQUIRED)
 find_package(X11 REQUIRED)
 
-# nlohmann/json
-FetchContent_Declare(nlohmann_json
-	URL https://github.com/nlohmann/json/releases/download/v3.12.0/json.tar.xz
-	DOWNLOAD_DIR ${CASPARCG_DOWNLOAD_CACHE}
-)
-FetchContent_MakeAvailable(nlohmann_json)
+# nlohmann/json - try system package first, fall back to fetching
+find_package(nlohmann_json 3.12.0 QUIET)
+if(NOT nlohmann_json_FOUND)
+    FetchContent_Declare(nlohmann_json
+        URL https://github.com/nlohmann/json/releases/download/v3.12.0/json.tar.xz
+        DOWNLOAD_DIR ${CASPARCG_DOWNLOAD_CACHE}
+    )
+    FetchContent_MakeAvailable(nlohmann_json)
+endif()
 
 # support for Ubuntu 22.04
 if (NOT TARGET OpenAL::OpenAL)

tools/linux/deb/ubuntu-jammy/debian/control

@@ -31,6 +31,7 @@ Build-Depends: cmake,
                libswscale-dev,
                libtbb-dev,
                libxrandr-dev,
+               nlohmann-json3-dev,
                pkg-config,
                txt2man,
                zlib1g-dev,

tools/linux/deb/ubuntu-noble/debian/control

@@ -31,6 +31,7 @@ Build-Depends: cmake,
                libswscale-dev,
                libtbb-dev,
                libxrandr-dev,
+               nlohmann-json3-dev,
                pkg-config,
                txt2man,
                zlib1g-dev,