Fix "-Wl,--dynamic-list" gdb/configure test

Fix "-Wl,--dynamic-list" gdb/configure test

The -Wl,--dynamic-list test is currently broken on Fedora 23, when you
configure with --with-python=python3.4.  We see:

 configure:13741: checking for the dynamic export flag
 configure:13796: gcc -o conftest -g3 -O0  -fno-strict-aliasing -DNDEBUG -fwrapv    -Wl,--dynamic-list=/home/pedro/gdb/mygit/src/gdb/proc-service.list conftest.c -ldl -lncurses -lm -ldl  -lpthread -ldl -lutil -lm -lpython3.4m -Xlinker -export-dynamic >&5
 conftest.c:182:30: fatal error: python3.4/Python.h: No such file or directory
 compilation terminated.
 configure:13796: $? = 1

The correct -I path is in PYTHON_CPPFLAGS:

 PYTHON_CPPFLAGS='-I/usr/include/python3.4m -I/usr/include/python3.4m'

(Other Python-related tests in the file are already doing this.)

gdb/ChangeLog:
2016-05-03  Pedro Alves  <palves@redhat.com>

	* configure.ac (checking for the dynamic export flag): Add
	$PYTHON_CPPFLAGS to CPPFLAGS.
	* configure: Regenerate.