build: add -std=c99 as default CFLAGS

We need to inject '-std=c99' because the reference implementation of blake2
is using C99 specific semantics.

------------------------------------------------------------------------------
crypto_generichash/blake2/ref/blake2b-ref.c: In function 'blake2b_init0':
crypto_generichash/blake2/ref/blake2b-ref.c:143:3: error: 'for' loop initial declarations are only allowed in C99 mode
crypto_generichash/blake2/ref/blake2b-ref.c:143:3: note: use option -std=c99 or -std=gnu99 to compile your code
crypto_generichash/blake2/ref/blake2b-ref.c: In function 'crypto_generichash_blake2b__init_param':
crypto_generichash/blake2/ref/blake2b-ref.c:155:3: error: 'for' loop initial declarations are only allowed in C99 mode
crypto_generichash/blake2/ref/blake2b-ref.c: In function 'crypto_generichash_blake2b__final':
crypto_generichash/blake2/ref/blake2b-ref.c:328:3: error: 'for' loop initial declarations are only allowed in C99 mode
------------------------------------------------------------------------------