Use FetchContent to include CPR

This also changes the logic to respect CPR's CPR_USE_SYSTEM_CURL. If that variable is not set, we configure CPR to use the system CURL if it is old enough to not cause issues and build its own otherwise. This allows for developing on recent systems but allows the easy use of a system libcurl when building in older distros for deployment.

Author: TheAssassin

lib/CMakeLists.txt

@@ -8,26 +8,56 @@ add_subdirectory(zlib)
 add_subdirectory(librcksum)
 add_subdirectory(libzsync)
 
-# external dependencies used as submodules
-
+# note: we use functions to import the dependencies to have separate scopes for the variables we apply on them
+# https://alexreinking.com/blog/building-a-dual-shared-and-static-library-with-cmake.html
 if(NOT USE_SYSTEM_CPR)
-    # for some reason we don't know, cpr wants to build shared libs by default
-    # this is not the default behavior of CMake, so we force this option off again
-    # unfortunately, we have to do this globally...
-    # long term goal is to replace the submodule in some way, e.g., by using prebuilt cpr binaries
-    set(BUILD_SHARED_LIBS OFF)
+    function(import_cpr)
+        # for some reason we don't know, cpr wants to build shared libs by default
+        # this is not the default behavior of CMake, so we force this option off again
+        # unfortunately, we have to do this globally...
+        # long term goal is to replace the submodule in some way, e.g., by using prebuilt cpr binaries
+        set(BUILD_SHARED_LIBS OFF)
+
+        # make sure that static libraries (especially built cURL libs) can be linked properly
+        set(CMAKE_POSITION_INDEPENDENT_CODE ON)
+
+        # there is no need to build our own libcurl
+        # in fact, building it as a static lib creates a whole circus of problems, and also takes time
+
+        # if a system provided installed libcurl is too "new", we want to have CPR build its own
+        # otherwise, we want to use the system provided library
+        # this way, we can build against a system-wide libcurl on older systems (e.g., Ubuntu focal) while allowing development on newer systems
+        # see
+        find_package(CURL)
+        if(CURL_FOUND)
+            message(STATUS "Found libcurl CURL_VERSION_STRING")
 
-    # make sure that static libraries (especially built cURL libs) can be linked properly
-    set(CMAKE_POSITION_INDEPENDENT_CODE ON)
+            if(NOT DEFINED CPR_USE_SYSTEM_CURL)
+                if (CURL_VERSION_STRING VERSION_LESS_EQUAL 7.80.0)
+                    message(STATUS "libcurl is old enough to work with CPR, so we use the system one (use -DCPR_USE_SYSTEM_CURL=OFF to change this behavior)")
+                    set(CPR_USE_SYSTEM_CURL ON)
+                else()
+                    message(WARNING "libcurl is too new to work with CPR, so we let CPR build its own (use -DCPR_USE_SYSTEM_CURL=ON to change this behavior)")
+                    set(CPR_USE_SYSTEM_CURL ON)
+                endif()
+            else()
+                message(WARNING "CPR_USE_SYSTEM_CURL is set to ${CPR_USE_SYSTEM_CURL} by the user")
+            endif()
+        endif()
 
-    # there is no need to build our own libcurl
-    # in fact, building it as a static lib creates a whole circus of problems, and also takes time
-    set(CPR_FORCE_USE_SYSTEM_CURL ON)
+        include(FetchContent)
+        FetchContent_Declare(cpr
+            GIT_REPOSITORY https://github.com/libcpr/cpr.git
+            # note: newer versions don't necessarily work with Ubuntu bionic's libcurl packages!
+            GIT_TAG 1.8.3
+        )
+        FetchContent_MakeAvailable(cpr)
 
-    add_subdirectory(cpr)
+        # since we want to have static builds of cpr, we have to enable -fPIC manually
+    #    set_property(TARGET cpr::cpr PROPERTY POSITION_INDEPENDENT_CODE ON)
+    endfunction()
 
-    # since we want to have static builds of cpr, we have to enable -fPIC manually
-    set_property(TARGET cpr PROPERTY POSITION_INDEPENDENT_CODE ON)
+    import_cpr()
 endif()
 
 if(NOT USE_SYSTEM_ARGS)