configure: silent MacOS clang warning gnu-folding-constant

On the specific fork of clang for MacOS, the compiler gets more
strict on the use of GNU folding constants, generating warnings
that halts the build of OpenOCD.
The GNU folding constants are highlighted by upstream clang only
for global variables. E.g.:
	const int len = 10;
	int array[len];
generates the warning on:
	clang -c x.c
	x.c:2:5: warning: variable length array folded to constant
		 array as an extension [-Wgnu-folding-constant]
	int array[len];
	    ^
Apparently only the MacOS fork generates warning for folded
constants inside a function. E.g.:
	int a(int *x);
	int b(void)
	{
		const int len = 10;
		int array[len];
		return a*array);
	}
does not return error even forcing -Wgnu-folding-constant on clang
upstream.

Current code triggers warning on the following lines due to the
size of the array being computed from a const variable:
	jtag/drivers/xds110.c:354         unsigned char data[max_data + 1];
	flash/nor/dw-spi.c:950            uint8_t buffer[buffer_size];
	flash/nor/dw-spi.c:980            uint8_t buffer[buffer_size];
	flash/nor/dw-spi.c:1034           uint8_t buffer[buffer_size];
	flash/nor/dw-spi.c:1065           uint8_t buffer[buffer_size];
	flash/nor/jtagspi.c:364           uint8_t ..., write_buffer[max], ...;
	flash/nor/stmqspi.c:778           char ..., output[(2 + max + 256) * 3 + 8];
	flash/nor/xcf.c:392               uint8_t reference[L];
	target/target.c:3303              char output[line_bytecnt * 4 + 1];
	target/semihosting_common.c:1787  char buf[buf_len];
	target/smp.c:59                   char hex_buffer[len * 2 + 1];
	target/smp.c:60                   uint8_t buffer[len];
	target/cortex_m.c:296             uint32_t r_vals[n_r32];
	target/cortex_m.c:297             uint32_t dhcsr[n_r32];
	target/x86_32_common.c:1337       char output[line_bytecnt * 4 + 1];
	target/riscv/riscv.c:2377         uint8_t buffer[length];
	target/xtensa/xtensa.c:536        uint8_t ops_padded[max_oplen];

While some of the const variable above could be replaced by macros,
for the majority of them I don't see such need, and the use of
const looks to me correct.

Silent the warning adding the clang flag -Wno-gnu-folding-constant.
The flag is not recognized by GCC, but it's silently ignored.

Change-Id: I1d452af115355bc4949b1616648fe6544cc48318
Reported-by: Frank Zeyda <frank.zeyda@gmail.com>
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/9431
Tested-by: jenkins
Reviewed-by: Marc Schink <dev@zapb.de>

configure.ac

@@ -763,6 +763,7 @@ AC_DEFINE([_GNU_SOURCE],[1],[Use GNU C library extensions (e.g. stdndup).])
 GCC_WARNINGS="-Wall -Wstrict-prototypes -Wformat-security -Wshadow"
 AS_IF([test "x${gcc_wextra}" = "xyes"], [
   GCC_WARNINGS="${GCC_WARNINGS} -Wextra -Wno-unused-parameter"
+  GCC_WARNINGS="${GCC_WARNINGS} -Wno-gnu-folding-constant"
   GCC_WARNINGS="${GCC_WARNINGS} -Wbad-function-cast"
   GCC_WARNINGS="${GCC_WARNINGS} -Wcast-align"
   GCC_WARNINGS="${GCC_WARNINGS} -Wredundant-decls"