dbcsr_api_c.F: broken with gcc-4.8.5 due to OPTIONAL in BIND(C) procedure

[dev-zero]

mpifort -c -O3 -g -fno-omit-frame-pointer -funroll-loops -ffast-math -fopenmp -D__MPI_VERSION=3 -D__parallel -D__SCALAPACK -D__STATM_TOTAL -ffree-form -std=f2003 -fimplicit-none -ffree-line-length-512 -Werror=aliasing -Werror=ampersand -Werror=c-binding-type -Werror=intrinsic-shadow -Werror=intrinsics-std -Werror=line-truncation -Werror=tabs -Werror=realloc-lhs-all -Werror=target-lifetime -Werror=underflow -Werror=unused-but-set-variable -Werror=unused-variable -Werror=unused-dummy-argument -Werror=conversion  -D__SHORT_FILE__="\"dbcsr_tensor_types.F\"" -I'/users/tiziano/work/dbcsr/dbcsr-fork/src/tensors/' -I'/users/tiziano/work/dbcsr/dbcsr-fork/src' dbcsr_tensor_types.F90 
/users/tiziano/work/dbcsr/dbcsr-fork/src/dbcsr_api_c.F:229.116:

a, transb, alpha, c_matrix_a, c_matrix_b, beta, c_matrix_c, retain_sparsity) &
                                                                           1
Error: TS 29113: Variable 'retain_sparsity' at (1) with OPTIONAL attribute in procedure 'c_dbcsr_multiply_d' which is BIND(C)

I guess this is intentional and requires 2008+TS support in the compiler, but there is no hint in the README about that fact.

[dev-zero]

Ok, I've updated the documentation. The bug actually occurred in an intermediate state while rebasing the develop branch and the -std=f2008ts was not added, sorry.