Ensure that the local headers have priority over installed headers

This fixes the build error I saw today:
phonenumbermatcher.cc:715:7: error: ‘GetNationalNumberGroupsForPattern’ was not declared in this scope
due to the compiler picking an older installed
phonenumbers/phonenumbermatcher.h header than the one under src.

This problem doesn't usually happen with cmake because normally
this would all be done from src/CMakeLists.txt with
set(CMAKE_INCLUDE_CURRENT_DIR ON), but that would require more
refactoring.

cpp/CMakeLists.txt

@@ -347,7 +347,7 @@ if (NOT WIN32)
   add_definitions ("-Wall -Werror")
 endif ()
 
-include_directories ("src")
+include_directories (BEFORE "src")
 
 if (${BUILD_STATIC_LIB} STREQUAL "ON")
   # Build a static library (without -fPIC).