[build] Fix bootstrap/51072: libitm not disabled without c++

[Eric Botcazou]

> Putting into gcc/cp/config-lang.in is a layering violation, it's true.  But
> until there's another instance that needs handling, it seems premature to
> build infrastructure to handle this.  And it's only one line after all...

I don't know exactly why, but this breaks bootstrap on some machines:

make[3]: Entering directory 
`/red.a/gnatmail-x/build-red/x86_64-linux/gnat/obj/libcpp'
g++  -I../../src/libcpp -I. -I../../src/libcpp/../include -I../../src/libcpp/include  -O2 -g -W -Wall -Wno-narrowing -Wwrite-strings -Wmissing-format-attribute -pedantic -Wno-long-long -Werror -fno-exceptions -fno-rtti -I../../src/libcpp -I. -I../../src/libcpp/../include -I../../src/libcpp/include  -c -o 
charset.o -MT 
charset.o -MMD -MP -MF .deps/charset.Tpo ../../src/libcpp/charset.c
cc1plus: error: unrecognized command line option "-Wno-narrowing"
make[3]: *** [charset.o] Error 1
make[3]: Leaving directory 
`/red.a/gnatmail-x/build-red/x86_64-linux/gnat/obj/libcpp'
make[2]: *** [all-stage2-libcpp] Error 2
make[2]: Leaving directory `/red.a/gnatmail-x/build-red/x86_64-linux/gnat/obj'
make[1]: *** [stage2-bubble] Error 2
make[1]: Leaving directory `/red.a/gnatmail-x/build-red/x86_64-linux/gnat/obj'

I have seen that on 3 different machines (i586, i686, x86-64).

-- 
Eric Botcazou