Use a macro to indicate C99 to the compiler

The [AC_PROG_CC_C99 macro](https://www.gnu.org/software/autoconf/manual/autoconf-2.64/html_node/C-Compiler.html) sets the complier to the C99 standard if the compiler does not default to such behaviour (albeit gnu99). This is better than specifically hardcoding flags

configure.ac

@@ -30,6 +30,7 @@ m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 
 # Checks for header files.
 AM_PROG_CC_C_O
+AC_PROG_CC_C99
 AC_CONFIG_HEADER(config.h)
 AC_CONFIG_HEADER(json_config.h)
 AC_HEADER_STDC
@@ -104,7 +105,6 @@ AC_SUBST(JSON_BSYMBOLIC_LDFLAGS)
 
 AX_APPEND_COMPILE_FLAGS([-Wall -Werror -Wno-error=deprecated-declarations])
 AX_APPEND_COMPILE_FLAGS([-Wextra -Wwrite-string -Wno-unused-parameter])
-AX_APPEND_COMPILE_FLAGS([-std=gnu99])
 AX_APPEND_COMPILE_FLAGS([-D_GNU_SOURCE -D_REENTRANT])
 
 AC_CONFIG_FILES([