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.