FindPkgConfig: set policies CMP0054 and CMP0057 to new

One may encounter warnings if FindPkgConfig is used in any project, even
indirectly, that has set any of these policies to old explicitely or requires
an older version.

Modules/FindPkgConfig.cmake

@@ -20,6 +20,10 @@ following variables will also be set:
 
 #]========================================]
 
+cmake_policy(PUSH)
+cmake_policy(SET CMP0054 NEW) # if() quoted variables not dereferenced
+cmake_policy(SET CMP0057 NEW) # if IN_LIST
+
 ### Common stuff ####
 set(PKG_CONFIG_VERSION 1)
 
@@ -770,3 +774,5 @@ Variables Affecting Behavior
 ### Local Variables:
 ### mode: cmake
 ### End:
+
+cmake_policy(POP)