configure: Really use local libfdt if the system one is too old

configure: Really use local libfdt if the system one is too old

QEMU requires libfdt version >= 1.4.2.
If the host has an older libfdt installed, the configure script will use
a (git cloned) local version.

Example with Debian 8:
    $ dpkg-query --showformat='${Version}\n' --show libfdt-dev
    1.4.0+dfsg-1
    $ ./configure
    [...]
    fdt support       yes          # from git submodule 'dtc'

If this case occurs, the linker will have 2 different libfdt available in
the library search path. The default behavior is to search the system path
first, then the local path.

Even if the configure script noticed the libfdt is too old and clone a more
recent locally, when linking the system library is selected first, and the
link process eventually fails:

      LINK    mips64el-softmmu/qemu-system-mips64el
    ../hw/core/loader-fit.o: In function `load_fit':
    /root/src/github.com/philmd/qemu/hw/core/loader-fit.c:278: undefined reference to `fdt_first_subnode'
    /root/src/github.com/philmd/qemu/hw/core/loader-fit.c:286: undefined reference to `fdt_next_subnode'
    /root/src/github.com/philmd/qemu/hw/core/loader-fit.c:277: undefined reference to `fdt_first_subnode'
    collect2: error: ld returned 1 exit status
    Makefile:201: recipe for target 'qemu-system-mips64el' failed
    make[1]: *** [qemu-system-mips64el] Error 1

QEMU already uses a kludge to enforce local CFLAGS before system ones for
libpixman and libfdt, add a similar kludge for the LDFLAGS to enforce using
the local libfdt.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180415230522.24404-2-f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>