bug#22124: libical: Non-deterministic build failures

[Mark H Weaver <mhw@netris.org>]

In the last month, libical has non-deterministically failed twice, once
on i686 and once on mips64el, with the same error in the same place:

--8<---------------cut here---------------start------------->8---
[ 19%] Building C object src/libicalss/CMakeFiles/icalss-static.dir/icalcalendar.c.o
cd /tmp/nix-build-libical-1.0.1.drv-0/build/src/libicalss && /gnu/store/8350z9d6ywpkx03n1il7b4xzvblx5gl2-gcc-4.9.3/bin/gcc  -DHAVE_CONFIG_H -Wno-deprecated -Wall -Wextra -Winit-self -Wmissing-include-dirs -Wunused -Wno-div-by-zero -Wundef -Wpointer-arith -Werror=return-type -Wunused-but-set-variable -Wlogical-op -Wsizeof-pointer-memaccess -O2 -g -DNDEBUG -I/tmp/nix-build-libical-1.0.1.drv-0/build -I/tmp/nix-build-libical-1.0.1.drv-0/libical-1.0.1/src -I/tmp/nix-build-libical-1.0.1.drv-0/build/src -I/tmp/nix-build-libical-1.0.1.drv-0/libical-1.0.1/src/libicalss -I/tmp/nix-build-libical-1.0.1.drv-0/build/src/libicalss -I/tmp/nix-build-libical-1.0.1.drv-0/libical-1.0.1/src/libical -I/tmp/nix-build-libical-1.0.1.drv-0/build/src/libical    -o CMakeFiles/icalss-static.dir/icalcalendar.c.o   -c /tmp/nix-build-libical-1.0.1.drv-0/libical-1.0.1/src/libicalss/icalcalendar.c
In file included from /tmp/nix-build-libical-1.0.1.drv-0/libical-1.0.1/src/libicalss/icalcalendar.h:32:0,
                 from /tmp/nix-build-libical-1.0.1.drv-0/libical-1.0.1/src/libicalss/icalcalendar.c:29:
/tmp/nix-build-libical-1.0.1.drv-0/build/src/libical/ical.h:1019:1: error: unknown type name ‘icalvalue’
 icalvalue* icalvalue_new(icalvalue_kind kind);
 ^
/tmp/nix-build-libical-1.0.1.drv-0/build/src/libical/ical.h:1019:26: error: unknown type name ‘icalvalue_kind’
 icalvalue* icalvalue_new(icalvalue_kind kind);
                          ^
/tmp/nix-build-libical-1.0.1.drv-0/build/src/libical/ical.h:1021:1: error: unknown type name ‘icalvalue’
 icalvalue* icalvalue_new_clone(const icalvalue* value);
 ^
--8<---------------cut here---------------end--------------->8---

Only the first few lines of the error output is shown above.  It is
followed by many lines of similar errors in 'ical.h'.

Restarting the builds later resulted in success.

Perhaps a parallel build issue?

     Mark