compilation: set use C99 mode by default

Before the patch, the default compiler compilation mode is used
(including C89, version dependent). But support the C89 dialect
in 2020 is strange. Accordingly, set C99 as preffered.

Author: LeonidVas

config.m4

@@ -1,4 +1,6 @@
 dnl config.m4 for extension tarantool
+dnl set C compiler to  C99 mode
+AC_PROG_CC_C99
 PHP_ARG_ENABLE(tarantool, for tarantool support,
 [  --enable-tarantool	Enable tarantool support])