Move CXX in ext/pdo_firebird after return()

This makes the CXX optional when pdo_firebird is disabled.

Author: petk

cmake/ext/pdo_firebird/CMakeLists.txt

@@ -1,6 +1,6 @@
 project(
   PhpExtensionPdoFirebird
-  LANGUAGES C CXX
+  LANGUAGES C
 )
 
 include(CheckLibraryExists)
@@ -27,6 +27,8 @@ if(NOT EXT_PDO_FIREBIRD)
   return()
 endif()
 
+enable_language(CXX)
+
 if(EXT_PDO_FIREBIRD_SHARED)
   add_library(php_pdo_firebird SHARED)
 else()