Otherwise we stumble upon file system limits when 'manual_cpp' has to be
used. This whole situation with the 'manual_cpp' is very unsatisfying...

The PGI compiler (of course) complained about a missing module
(elpa_generated_fortran_interfaces.mod) when compiling the test
programs.

It is true (in a way) that some part of this module is indeed necessary,
as the public-facing function signatures have arguments that are
referring to those three functions in their type:

  elpa_strerr_c(elpa_error)
  elpa_int_value_to_string_c(name, value, string)
  elpa_int_value_to_strlen_c(name, value)

Thus, for these three we create another header prefix,

  !pf>

for Fortran definitions that should be public. Those are included in
elpa_api.F90.

It is not quite clear to me why GCC complains here when
WANT_SINGLE_PRECISION_REAL is not defined, as the variables are then not
used _at all_.

Nonetheless, this again prevents build on the SuSE OBS, so fix it to
make the compiler happy

Another blocker for the SuSE build server: Even though the variable is
later not actually used by the function, a recent GCC complains about
the use of an uninitialized value for that variable.

This prevents build on the OBS, as they use compiliation flags that
detect such behaviour.

Error message there:

 ../test/C/driver/legacy_interface/legacy_real_driver_c_version.c:193:12: warning: 'THIS_REAL_ELPA_KERNEL_API' is used uninitialized in this function [-Wuninitialized]
     success = elpa_solve_evp_real_double(na, nev, a, na_rows, ev, z, na_rows, nblk, na_cols, mpi_comm_rows, mpi_comm_cols, my_mpi_comm_world, THIS_REAL_ELPA_KERNEL_API, useQr, useGPU,

 [...]

 I: Program is using uninitialized variables.
    Note the difference between "is used" and "may be used"
 W: elpa uninitialized-variable ../test/C/driver/legacy_interface/legacy_real_driver_c_version.c:193

Recent GCC complain that an assignment involving a transfer statement
might potentially overflow the destination buffer. This prevented a
build on the SuSE build server.

Replaced this with a proper C string pointer.

Error message there:

 ../src/elpa_driver/legacy_interface/./elpa_driver_c_interface_template.X90:117:0:

        methodFortran(1:charCount) = transfer(method(1:charCount), methodFortran)

 Warning: '__builtin_memset': writing between 1 and 2147483640 bytes into a region of size 0 overflows the destination [-Wstringop-overflow=]

 [...]

 I: Statement might potentially overflow a destination buffer, where a size larger
     than the actual buffer was specified
 E: elpa destbufferoverflow Warning: '__builtin_memset'

There was an error in the strip_width calculation leading to wrong
results for certain values of 'nev'. This is still not properly
investigated, should be checked again.

Exchange _mm512_load_pd()/_mm512_load_ps() of stack variables with
_mm512_set_pd, as the stack variables might not have proper alignment!

Most of the test programs for the new interface were all essentially
copy&pasted from test.F90, now all of those directly use test.F90 with
appropriate preprocessor flags

- Remove all use of ELPA internal modules, the test programs now
  only use the public ELPA API. This is now enforced, by hiding the
  private modules

- Prefix all test internal modules with "test_" to make it really
  clear that these modules are not to be used by users.