For some reason, presumably historical, the `install-gnatlib' target for
the default multilib is invoked twice, once via the `ada.install-common'
target in `gcc/ada/gcc-interface/Make-lang.in' invoked from gcc/ and
again via the `install-libada' target in libada/.

Apart from doing the same twice this is actually harmful in sufficiently
parallelized `make' invocation, as the removal of old files performed
within the `install-gnatlib' recipe in the former case actually races
with the installation of new files done in the latter case, causing the
recipe to fail and abort, however non-fatally, having not completed the
installation of all the built files needed for the newly-built compiler
to work correctly.

This can be observed with a native `x86_64-linux-gnu' bootstrap:

make[4]: Entering directory '.../gcc/ada'
rm -rf .../lib/gcc/x86_64-linux-gnu/10.0.0/adalib
rm: cannot remove '.../lib/gcc/x86_64-linux-gnu/10.0.0/adalib': Directory not empty
make[4]: *** [gcc-interface/Makefile:512: install-gnatlib] Error 1
make[4]: Leaving directory '.../gcc/ada'
make[3]: *** [.../gcc/ada/gcc-interface/Make-lang.in:853: install-gnatlib] Error 2
make[2]: [.../gcc/ada/gcc-interface/Make-lang.in:829: ada.install-common] Error 2 (ignored)

which then causes missing files to be reported when an attempt is made
to use the newly-installed non-functional compiler to build a
`riscv-linux-gnu' cross-compiler:

(cd ada/bldtools/sinfo; gnatmake -q xsinfo ; ./xsinfo sinfo.h )
error: "ada.ali" not found, "ada.ads" must be compiled
error: "s-memory.ali" not found, "s-memory.adb" must be compiled
gnatmake: *** bind failed.
/bin/sh: ./xsinfo: No such file or directory
make[2]: *** [.../gcc/ada/Make-generated.in:45: ada/sinfo.h] Error 127
make[2]: Leaving directory '.../gcc'
make[1]: *** [Makefile:4369: all-gcc] Error 2
make[1]: Leaving directory '...'
make: *** [Makefile:965: all] Error 2

Depending on timing `.../lib/gcc/x86_64-linux-gnu/10.0.0/adainclude' may
cause an installation failure instead and the resulting compiler may be
non-functional in a different way.

Only invoke `install-gnatlib' from within gcc/ then if a legacy build
process is being used with libada disabled and gnatlib built manually
with `make -C gcc gnatlib'.

	gcc/
	* Makefile.in (gnat_install_lib): New variable.
	* configure.ac: Substitute it.
	* configure: Regenerate.

	gcc/ada/
	* gcc-interface/Make-lang.in (ada.install-common): Split into...
	(gnat-install-tools, gnat-install-lib): ... these.

From-SVN: r276422