bug#41213: boost-for-mysql fails to build

[Jonathan Brielmaier]

boost-for-mysql fails to build on current master with following error.

```
starting phase `configure'
Backtrace:
          10 (primitive-load "/gnu/store/bsfksp6c63zj3ynx46ck87sip7a…")
In ice-9/eval.scm:
   191:35  9 (_ _)
In guix/build/gnu-build-system.scm:
    838:2  8 (gnu-build #:source _ #:outputs _ #:inputs _ #:phases . #)
In ice-9/boot-9.scm:
  1736:10  7 (with-exception-handler _ _ #:unwind? _ # _)
In srfi/srfi-1.scm:
   857:16  6 (every1 #<procedure 7ffff4b99040 at guix/build/gnu-bui…> …)
In guix/build/gnu-build-system.scm:
   847:30  5 (_ _)
In ice-9/eval.scm:
    619:8  4 (_ #(#(#(#<directory (guile-user) 7ffff5bb9f00>) # #) …))
In srfi/srfi-1.scm:
    634:9  3 (for-each #<procedure substitute-one-file (a)> _)
In guix/build/utils.scm:
   736:30  2 (with-atomic-file-replacement "tools/build/src/engine/…" …)
In unknown file:
           1 (stat "tools/build/src/engine/execunix.cpp" #<undefined>)
In ice-9/boot-9.scm:
  1669:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1669:16: In procedure raise-exception:
In procedure stat: No such file or directory:
"tools/build/src/engine/execunix.cpp"
builder for
`/gnu/store/sk133zc1fbs5352a3k9lzmfg2930rjjx-boost-1.59.0.drv' failed
with exit code 1
build of /gnu/store/sk133zc1fbs5352a3k9lzmfg2930rjjx-boost-1.59.0.drv failed
View build log at
'/var/log/guix/drvs/sk/133zc1fbs5352a3k9lzmfg2930rjjx-boost-1.59.0.drv.bz2'.
cannot build derivation
`/gnu/store/bjn6im95g4669731sfzd8vlyj5742ir3-mysql-5.7.27.drv': 1
dependencies couldn't be built
guix build: error: build of
`/gnu/store/bjn6im95g4669731sfzd8vlyj5742ir3-mysql-5.7.27.drv' failed
```

"tools/build/src/engine/execunix.cpp" is required for boost@1.72 in the
configure phase. boost-for-mysql inherits from.

I don't know how to properly fix that...