Bootstrapping core-updates fails with mpc-1.2.1dev

[Andreas Enge <andreas@enge.fr>]

Hello,

after the release of mpc-1.2.0, I tried to update the version in core-updates.
More precisely, I used a tarball of the current master under the name of
mpc-1.2.1dev, with an additional commit. Then bootstrapping fails.

For someone with bayfront access, the build log is in
   /var/log/guix/drvs/ay/qa5whpm2nq3667ff69xzqvn9119b8g-gcc-cross-boot0-7.5.0.drv.gz

It ends with this:
...
g++ -fno-PIE -c  -DIN_GCC_FRONTEND -g -O2 -DIN_GCC  -DCROSS_DIRECTORY_STRUCTURE   -fno-exceptions -fno-rtti -fasynchronous-unwind-tables -W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual -Wmissing-format-attribute -Woverloaded-virtual -pedantic -Wno-long-long -Wno-variadic-macros -Wno-overlength-strings   -DHAVE_CONFIG_H -I. -Ic -I../../gcc-7.5.0/gcc -I../../gcc-7.5.0/gcc/c -I../../gcc-7.5.0/gcc/../include -I../../gcc-7.5.0/gcc/../libcpp/include -I/tmp/guix-build-gcc-cross-boot0-7.5.0.drv-0/build/./gmp -I/tmp/guix-build-gcc-cross-boot0-7.5.0.drv-0/gcc-7.5.0/gmp -I/tmp/guix-build-gcc-cross-boot0-7.5.0.drv-0/build/./mpfr/src -I/tmp/guix-build-gcc-cross-boot0-7.5.0.drv-0/gcc-7.5.0/mpfr/src  -I../../gcc-7.5.0/gcc/../libdecnumber -I../../gcc-7.5.0/gcc/../libdecnumber/dpd -I../libdecnumber -I../../gcc-7.5.0/gcc/../libbacktrace   -o c/c-typeck.o -MT c/c-typeck.o -MMD -MP -MF c/.deps/c-typeck.TPo ../../gcc-7.5.0/gcc/c/c-typeck.c
In file included from ../../gcc-7.5.0/gcc/c/c-decl.c:53:0:
../../gcc-7.5.0/gcc/builtins.h:23:17: fatal error: mpc.h: No such file or directory
 #include <mpc.h>
                 ^
compilation terminated.
make[2]: *** [Makefile:1099: c/c-decl.o] Error 1
...

Does this ring a bell? I do not quite see how this can be related to the mpc
version used for bootstrapping; and the phase unpack-gmp&co is carried out
and does unpack my mpc-1.2.1dev.tar.gz after gmp and mpfr.

What is strange in the compiler line above is that gmp and mpfr are
referenced by -I directives, but mpc is not.

Is there anything special with gmp-7.5.0 that prevents it from being compiled
with a newer mpc? The build log also shows that gmp and mpfr are compiled in
their respective subdirectories, but mpc is not.

Andreas