ExternalProject: Fix file download script with CMAKE_TLS_{VERIFY,CAINFO}

Avoid generating incorrect code such as

    set(CMAKE_TLS_VERIFY set(CMAKE_TLS_VERIFY 1))

when one of these variables is set in the calling project.

Modules/ExternalProject.cmake

@@ -859,12 +859,15 @@ function(_ep_write_downloadfile_script script_filename remote local timeout no_p
     set(hash_check "")
   endif()
 
+  set(tls_verify_code "")
+  set(tls_cainfo_code "")
+
   # check for curl globals in the project
   if(DEFINED CMAKE_TLS_VERIFY)
-    set(tls_verify "set(CMAKE_TLS_VERIFY ${CMAKE_TLS_VERIFY})")
+    set(tls_verify_code "set(CMAKE_TLS_VERIFY ${CMAKE_TLS_VERIFY})")
   endif()
   if(DEFINED CMAKE_TLS_CAINFO)
-    set(tls_cainfo "set(CMAKE_TLS_CAINFO \"${CMAKE_TLS_CAINFO}\")")
+    set(tls_cainfo_code "set(CMAKE_TLS_CAINFO \"${CMAKE_TLS_CAINFO}\")")
   endif()
 
   # now check for curl locals so that the local values
@@ -873,12 +876,12 @@ function(_ep_write_downloadfile_script script_filename remote local timeout no_p
   # check for tls_verify argument
   string(LENGTH "${tls_verify}" tls_verify_len)
   if(tls_verify_len GREATER 0)
-    set(tls_verify "set(CMAKE_TLS_VERIFY ${tls_verify})")
+    set(tls_verify_code "set(CMAKE_TLS_VERIFY ${tls_verify})")
   endif()
   # check for tls_cainfo argument
   string(LENGTH "${tls_cainfo}" tls_cainfo_len)
   if(tls_cainfo_len GREATER 0)
-    set(tls_cainfo "set(CMAKE_TLS_CAINFO \"${tls_cainfo}\")")
+    set(tls_cainfo_code "set(CMAKE_TLS_CAINFO \"${tls_cainfo}\")")
   endif()
 
   file(WRITE ${script_filename}
@@ -887,8 +890,8 @@ function(_ep_write_downloadfile_script script_filename remote local timeout no_p
      dst='${local}'
      timeout='${timeout_msg}'\")
 
-${tls_verify}
-${tls_cainfo}
+${tls_verify_code}
+${tls_cainfo_code}
 
 file(DOWNLOAD
   \"${remote}\"