Fix CPR variable name

After trying to use CPR 1.10.0 first, which uses a new variable name, we had to move back to 1.8.3, which works well with the system libcurl on Ubuntu bionic.

Author: TheAssassin

lib/CMakeLists.txt

@@ -30,15 +30,15 @@ if(NOT USE_SYSTEM_CPR)
         # see
         find_package(CURL)
         if(CURL_FOUND)
-            message(STATUS "Found libcurl CURL_VERSION_STRING")
+            message(STATUS "Found libcurl ${CURL_VERSION_STRING}")
 
-            if(NOT DEFINED CPR_USE_SYSTEM_CURL)
+            if(NOT DEFINED CPR_FORCE_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)
+                    message(STATUS "libcurl is old enough to work with CPR, so we use the system one (use -DCPR_FORCE_USE_SYSTEM_CURL=OFF to change this behavior)")
+                    set(CPR_FORCE_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)
+                    message(WARNING "libcurl is too new to work with CPR, so we let CPR build its own (use -DCPR_FORCE_USE_SYSTEM_CURL=ON to change this behavior)")
+                    set(CPR_FORCE_USE_SYSTEM_CURL ON)
                 endif()
             else()
                 message(WARNING "CPR_USE_SYSTEM_CURL is set to ${CPR_USE_SYSTEM_CURL} by the user")