Add C99 compilation option
If the gcc compiler is too old (GCC 4.8.5 for instance) the pip installation failed with the following message :
In file included from /tmp/pip-install-nsjlvfq3/ltfatpy/ltfat_C_kernel/ltfat.h:3:0,
from /tmp/pip-install-nsjlvfq3/ltfatpy/ltfat_C_kernel/dgt_fac.c:1:
/tmp/pip-install-nsjlvfq3/ltfatpy/ltfat_C_kernel/dgt_fac.c: In function ‘dgt_long_done_d’:
/tmp/pip-install-nsjlvfq3/ltfatpy/ltfat_C_kernel/config.h:39:4: erreur: ‘for’ loop initial declarations are only allowed in C99 mode
for(size_t ii=0;ii<len;ii++) \
^
The C99 compilation option should probably be added in the CMakeList.txt to close this issue.