Pass PKG_CONFIG_PATH down from top-level Makefile

[Sending to binutils, gdb-patches and gcc-patches, since it touches the
top-level Makefile/configure]

I have my debuginfod library installed in a non-standard location
(/opt/debuginfod), which requires me to set
PKG_CONFIG_PATH=/opt/debuginfod/lib/pkg-config.  If I just set it during
configure:

    $ PKG_CONFIG_PATH=/opt/debuginfod/lib/pkg-config ./configure --with-debuginfod
    $ make

or

    $ ./configure --with-debuginfod PKG_CONFIG_PATH=/opt/debuginfod/lib/pkg-config
    $ make

Then PKG_CONFIG_PATH is only present (and ignored) during the top-level
configure.  When running make (which runs gdb's and binutils'
configure), PKG_CONFIG_PATH is not set, which results in their configure
script not finding the library:

    checking for libdebuginfod >= 0.179... no
    configure: error: "--with-debuginfod was given, but libdebuginfod is missing or unusable."

Change the top-level configure/Makefile system to capture the value
passed when configuring the top-level and pass it down to
subdirectories (similar to CFLAGS, LDFLAGS, etc).

I don't know much about the top-level build system, so I really don't
know if I did this correctly.  The changes are:

 - Use AC_SUBST(PKG_CONFIG_PATH) in configure.ac, so that
   @PKG_CONFIG_PATH@ gets replaced with the actual PKG_CONFIG_PATH value
   in config files (i.e. Makefile)
 - Add a PKG_CONFIG_PATH Makefile variable in Makefile.tpl, initialized
   to @PKG_CONFIG_PATH@
 - Add PKG_CONFIG_PATH to HOST_EXPORTS in Makefile.tpl, which are the
   variables set when running the sub-configures

I initially added PKG_CONFIG_PATH to flags_to_pass, in Makefile.def, but
I don't think it's needed.  AFAIU, this defines the flags to pass down
when calling "make" in subdirectories.  We only need PKG_CONFIG_PATH to
be passed down during configure.  After that, it's captured in
gdb/config.status, so even if a "make" causes a re-configure later
(because gdb/configure has changed, for example), the PKG_CONFIG_PATH
value will be remembered.

Change-Id: I91138dfca41c43b05e53e445f62e4b27882536bf

ChangeLog:

	* configure.ac: Add AC_SUBST(PKG_CONFIG_PATH).
	* configure: Re-generate.
	* Makefile.tpl (HOST_EXPORTS): Pass PKG_CONFIG_PATH.
	(PKG_CONFIG_PATH): New.
	* Makefile.in: Re-generate.