configure.ac: fix __progname check with gcc >= 14

[ffontaine]

Include stdio.h to avoid that __progname check wrongly returns that the function is unavailable with gcc >= 14 which enables -Werror=implicit-function-declaration (https://gcc.gnu.org/gcc-14/porting_to.html):

configure:5231: checking if libc defines __progname configure:5249: /home/autobuild/autobuild/instance-7/output-1/host/bin/sh4-buildroot-linux-musl-gcc -o conftest -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os -g0  -static -Wall -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -static conftest.c  >&5 conftest.c: In function 'main':
conftest.c:73:27: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   73 |  extern char *__progname; printf("%s", __progname);
      |                           ^~~~~~
conftest.c:1:1: note: include '<stdio.h>' or provide a declaration of 'printf'
    1 | /* confdefs.h */
conftest.c:73:27: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   73 |  extern char *__progname; printf("%s", __progname);
      |                           ^~~~~~
conftest.c:73:27: note: include '<stdio.h>' or provide a declaration of 'printf'

resulting in the following static build failure:

/home/autobuild/autobuild/instance-7/output-1/host/lib/gcc/sh4-buildroot-linux-musl/14.1.0/../../../../sh4-buildroot-linux-musl/bin/ld: /home/autobuild/autobuild/instance-7/output-1/host/sh4-buildroot-linux-musl/sysroot/lib/libc.a(libc.o):(.bss.__progname+0x0): multiple definition of `__progname'; autossh.o:(.bss+0x120): first defined here

Fixes:

• http://autobuild.buildroot.org/results/5d80a8096d0f5529cbd86903a74f3bab3d230f4a