clang build fails with `../grub-core/lib/gnulib/regex.h:682:20: error: variable length array used [-Werror,-Wvla]`

[Paul Menzel]

Dear GRUB folks,


On Debian sid/unstable clang 13.0.1 fails to build GRUB due to a problem 
in Gnulib:

```
$ git log --oneline -1 --no-decorate
7c316e183 term/efi/console: Do not set cursor until the first text output
$ ./configure --with-platform=coreboot CC=clang && make
[…]
clang -DHAVE_CONFIG_H -I. -I..  -Wall -W  -DGRUB_MACHINE_COREBOOT=1 
-DGRUB_MACHINE=I386_COREBOOT -m32 -msoft-float -Xclang -msoft-float 
-Xclang -no-implicit-float -nostdinc -isystem 
/usr/lib/llvm-13/lib/clang/13.0.1/include -I../include -I../include 
-DGRUB_FILE=\"commands/regexp.c\" -I. -I. -I.. -I.. -I../include 
-I../include -I../grub-core/lib/libgcrypt-grub/src/ 
-I../grub-core/lib/posix_wrap -I../grub-core/lib/gnulib 
-I../grub-core/lib/gnulib  -D_FILE_OFFSET_BITS=64 -std=gnu99 -fno-common 
-Os -m32 -Wall -W -Wshadow -Wpointer-arith -Wundef -Wchar-subscripts 
-Wcomment -Wdeprecated-declarations -Wdisabled-optimization 
-Wdiv-by-zero -Wfloat-equal -Wformat-extra-args -Wformat-security 
-Wformat-y2k -Wimplicit -Wimplicit-function-declaration -Wimplicit-int 
-Wmain -Wmissing-braces -Wmissing-format-attribute -Wmultichar 
-Wparentheses -Wreturn-type -Wsequence-point -Wshadow -Wsign-compare 
-Wswitch -Wtrigraphs -Wunknown-pragmas -Wunused -Wunused-function 
-Wunused-label -Wunused-parameter -Wunused-value  -Wunused-variable 
-Wwrite-strings -Wnested-externs -Wstrict-prototypes -g 
-Wredundant-decls -Wmissing-prototypes -Wmissing-declarations  -Wextra 
-Wattributes -Wendif-labels -Winit-self -Wint-to-pointer-cast 
-Winvalid-pch -Wmissing-field-initializers -Wnonnull -Woverflow -Wvla 
-Wpointer-to-int-cast -Wstrict-aliasing -Wvariadic-macros 
-Wvolatile-register-var -Wpointer-sign -Wmissing-include-dirs 
-Wmissing-prototypes -Wmissing-declarations -Wformat=2 -march=i386 
-falign-functions=1 -freg-struct-return -mno-mmx -mno-sse -mno-sse2 
-mno-sse3 -mno-3dnow -fno-dwarf2-cfi-asm -mno-stack-arg-probe 
-fno-asynchronous-unwind-tables -fno-unwind-tables -fno-ident -Qn 
-Qunused-arguments -fno-stack-protector -Werror   -ffreestanding 
-fno-builtin -Wno-undef -Wno-sign-compare -Wno-unused 
-Wno-unused-parameter -Wno-redundant-decls -Wno-unreachable-code 
-Wno-conversion   -MT commands/regexp_module-regexp.o -MD -MP -MF 
commands/.deps-core/regexp_module-regexp.Tpo -c -o 
commands/regexp_module-regexp.o `test -f 'commands/regexp.c' || echo 
'./'`commands/regexp.c
In file included from commands/regexp.c:28:
../grub-core/lib/gnulib/regex.h:682:20: error: variable length array 
used [-Werror,-Wvla]
                                        _REGEX_NELTS (__nmatch)],
                                                      ^~~~~~~~
../grub-core/lib/gnulib/regex.h:528:27: note: expanded from macro 
'_REGEX_NELTS'
#  define _REGEX_NELTS(n) n
                          ^
1 error generated.
make[3]: *** [Makefile:41321: commands/regexp_module-regexp.o] Fehler 1
```

I tried to use latest Gnulib (not the one defined in `bootstrap.conf`), 
but the error remains.

How to deal with that, if I am unsure how to fix it? Report it to the 
Gnulib folks?


Kind regards,

Paul