This commit is out of sequence, it really belongs between commits
1509833141 (Automatic update of
common submodule) and dd26f378b0
(configure: Fix the CELT checks to ...)
This first checks what is required for ISO C99 support and sets the relevant
compiler parameters and if no C99 compiler is found, it checks for a
C89 compiler. This enables us to check for and use C89/C99 functions
that gcc hides from us without the correct compiler parameters.