[tlbuild] Failure to build mpfr from TL: 'gmp.h' file not found

[Akira Kakuto]

> The file gmp.h is present under
>    /path/to/texlive/source/Work/libs/gmp/gmp.h
> but not under gmp/include/gmp.h as the include path suggests.

Usually there are lines like:
... ...
Entering directory '/path/to/texlive/source/Work/libs/gmp/include'
ln -s ../gmp.h gmp.h
... ...

in build messages for gmp.

Best,
Akira