Move custom headers into their own directory

--- Makefile.am.orig	2017-09-23 23:21:02 UTC
+++ Makefile.am
@@ -208,7 +208,10 @@ pkginclude_internal_HEADERS = \
   include/gtest/internal/gtest-port-arch.h \
   include/gtest/internal/gtest-string.h \
   include/gtest/internal/gtest-tuple.h \
-  include/gtest/internal/gtest-type-util.h \
+  include/gtest/internal/gtest-type-util.h
+
+pkginclude_internal_customdir = $(pkgincludedir)/internal/custom
+pkginclude_internal_custom_HEADERS = \
   include/gtest/internal/custom/gtest.h \
   include/gtest/internal/custom/gtest-port.h \
   include/gtest/internal/custom/gtest-printers.h