undefined behaviour due to LAPACK prototypes missing "hidden" variables required by Fortran

The prototypes for Fortran LAPACK functions appear to be incomplete in Dlib. This can lead to undefined behaviour with recent releases of gcc due to more aggressive compiler optimisations.

According to gfortran passing conventions (followed by other fortran compilers as well), every `char*` argument also has an associated "hidden" argument which specifies the number of characters. The type of the "hidden" argument may vary across compilers and/or compiler versions. The position of each "hidden" argument is also compiler dependent, though seems to be typically tacked onto the end of the function definition.
https://gcc.gnu.org/onlinedocs/gfortran/Argument-passing-conventions.html

As an example in Dlib, the current prototype for `dsyev` in
https://github.com/davisking/dlib/blob/master/dlib/matrix/lapack/syev.h
is defined as:
`void DLIB_FORTRAN_ID(dsyev) (const char *jobz, const char *uplo, const integer *n, double *a, const integer *lda, double *w, double *work, const integer *lwork, integer *info);`

But it probably should be:
`void DLIB_FORTRAN_ID(dsyev) (const char *jobz, const char *uplo, const integer *n, double *a, const integer *lda, double *w, double *work, const integer *lwork, integer *info, blas_len jobz_len, blas_len uplo_len);`

where `blas_len` is typically a 32 bit unsigned int on 32 bit platforms, and 64 bit unsigned int on 64 bit platforms. (This does not apply to gcc versions <= 7, where it is always `int`).

Avoiding the use of the blas_len arguments appeared to work, but recently gcc has started to optimise more heavily, leading to stack overwrites when these arguments are not used.

The same bug affects Armadillo, R, and a lot of other software in C and C++ that uses LAPACK:
* https://gitlab.com/conradsnicta/armadillo-code/issues/123
* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90329
* https://gcc.gnu.org/ml/gcc-patches/2019-05/msg00915.html
* https://github.com/Reference-LAPACK/lapack/issues/339

CC: @dodomorandi