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

[Jonathan Brielmaier]

`tools/build/src/engine/execunix.cpp` was named
`tools/build/src/engine/execunix.c` before. So the following patch
allows to build boost@1.59 again.

```
diff --git a/gnu/packages/boost.scm b/gnu/packages/boost.scm
index 2f2ca289ab..d44b534939 100644
--- a/gnu/packages/boost.scm
+++ b/gnu/packages/boost.scm
@@ -120,10 +120,11 @@
              (let ((icu (assoc-ref inputs "icu4c"))
                    (python (assoc-ref inputs "python"))
                    (out (assoc-ref outputs "out")))
-               (substitute* '("libs/config/configure"
-                              "libs/spirit/classic/phoenix/test/runtest.sh"
-                              "tools/build/src/engine/execunix.cpp"
-                              "tools/build/src/engine/Jambase")
+               (substitute* (append
+                             (find-files "tools/build/src/engine/"
"execunix\\.c.*")
+                             '("libs/config/configure"
+
"libs/spirit/classic/phoenix/test/runtest.sh"
+                               "tools/build/src/engine/Jambase"))
                  (("/bin/sh") (which "sh")))

                (setenv "SHELL" (which "sh"))
```

Although it fails with
```
libs/python/src/converter/builtin_converters.cpp: In function ‘void*
boost::python::converter::{anonymous}::convert_to_cstring(PyObject*)’:
libs/python/src/converter/builtin_converters.cpp:51:35: error: invalid
conversion from ‘const void*’ to ‘void*’ [-fpermissive]
       return PyUnicode_Check(obj) ? _PyUnicode_AsString(obj) : 0;

    "g++"  -ftemplate-depth-128 -O3 -finline-functions -Wno-inline -Wall
-pthread -fPIC -m64  -DBOOST_ALL_NO_LIB=1 -DBOOST_PYTHON_SOURCE -DNDEBUG
 -I"."
-I"/gnu/store/q9rm8h9imazsq2c4qiv2yjpvlvliywqb-python-3.8.2/include/python3.8"
-c -o
"bin.v2/libs/python/build/gcc-7.5.0/release/threading-multi/converter/builtin_converters.o"
"libs/python/src/converter/builtin_converters.cpp"
```