FindCURL: Fix version string in case of multiple curl installations

On systems where there are two versions of libcurl installed
CURL_VERSION_STRING is always set to the version found by the
`pkg_check_modules(PC_CURL QUIET libcurl)`.

But CURL_VERSION_STRING should match the libs and include paths found by
`find_path` and `find_library` (more specifically the path where the
header is found, because that where the version is parsed from.

So make sure that the version is always parsed from the header file, use
the output of pkg_check_modules only as HINT for find_path /
find_library.

Modules/FindCURL.cmake

@@ -82,7 +82,6 @@ find_package(PkgConfig QUIET)
 if(PKG_CONFIG_FOUND)
   pkg_check_modules(PC_CURL QUIET libcurl)
   if(PC_CURL_FOUND)
-    set(CURL_VERSION_STRING ${PC_CURL_VERSION})
     pkg_get_variable(CURL_SUPPORTED_PROTOCOLS libcurl supported_protocols)
     pkg_get_variable(CURL_SUPPORTED_FEATURES libcurl supported_features)
   endif()
@@ -122,7 +121,7 @@ if(NOT CURL_LIBRARY)
   select_library_configurations(CURL)
 endif()
 
-if(CURL_INCLUDE_DIR AND NOT CURL_VERSION_STRING)
+if(CURL_INCLUDE_DIR)
   foreach(_curl_version_header curlver.h curl.h)
     if(EXISTS "${CURL_INCLUDE_DIR}/curl/${_curl_version_header}")
       file(STRINGS "${CURL_INCLUDE_DIR}/curl/${_curl_version_header}" curl_version_str REGEX "^#define[\t ]+LIBCURL_VERSION[\t ]+\".*\"")