Build failure with GCC 15 (defaults to -std=gnu23)

[Sam James]

Hi,

GCC 15 (in development) defaults to -std=gnu23. This breaks one of
gmp-6.3.0's configure tests:
"""
$ ./configure
checking build system type... zen2-pc-linux-gnu
checking host system type... zen2-pc-linux-gnu
checking for a BSD-compatible install... /usr/bin/install -c
checking whether build environment is sane... yes
checking for a thread-safe mkdir -p... /usr/bin/mkdir -p
checking for gawk... gawk
checking whether make sets $(MAKE)... yes
checking whether make supports nested variables... yes
checking whether to enable maintainer-specific portions of Makefiles... no
checking ABI=64
checking compiler gcc -O2 -pedantic -fomit-frame-pointer -m64 ... no, long long reliability test 1
checking whether cc is gcc... yes
checking compiler cc -O2 -pedantic -fomit-frame-pointer -m64 ... no, long long reliability test 1
checking ABI=x32
checking compiler gcc -O2 -pedantic -fomit-frame-pointer -mx32 ... no
checking whether cc is gcc... yes
checking compiler cc -O2 -pedantic -fomit-frame-pointer -mx32 ... no
checking ABI=32
checking compiler gcc -m32 -O2 -pedantic -fomit-frame-pointer ... no, long long reliability test 1
checking compiler gcc -O2 -pedantic -fomit-frame-pointer ... no, long long reliability test 1
checking compiler icc -no-gcc ... no
checking whether cc is gcc... yes
checking compiler cc -m32 -O2 -pedantic -fomit-frame-pointer ... no, long long reliability test 1
checking compiler cc -O2 -pedantic -fomit-frame-pointer ... no, long long reliability test 1
configure: error: could not find a working compiler, see config.log for details
"""

The relevant snippet from config.log is as follows:
"""
configure:6585: cc -O2 -pedantic -fomit-frame-pointer  conftest.c >&5
conftest.c: In function 'f':
conftest.c:12:48: error: too many arguments to function 'g'
   12 | for(i=0;i<1;i++){if(e(got,got,9,d[i].n)==0)h();g(i,d[i].src,d[i].n,got,d[i].want,9);if(d[i].n)h();}}
      |                                                ^
conftest.c:7:6: note: declared here
    7 | void g(){}
      |      ^
configure:6588: $? = 1
failed program was:
/* The following provokes a segfault in the compiler on powerpc-apple-darwin.
   Extracted from tests/mpn/t-iord_u.c.  Causes Apple's gcc 3.3 build 1640 and
   1666 to segfault with e.g., -O2 -mpowerpc64.  */

#if defined (__GNUC__) && ! defined (__cplusplus)
typedef unsigned long long t1;typedef t1*t2;
void g(){}
void h(){}
static __inline__ t1 e(t2 rp,t2 up,int n,t1 v0)
{t1 c,x,r;int i;if(v0){c=1;for(i=1;i<n;i++){x=up[i];r=x+1;rp[i]=r;}}return c;}
void f(){static const struct{t1 n;t1 src[9];t1 want[9];}d[]={{1,{0},{1}},};t1 got[9];int i;
for(i=0;i<1;i++){if(e(got,got,9,d[i].n)==0)h();g(i,d[i].src,d[i].n,got,d[i].want,9);if(d[i].n)h();}}
#else
int dummy;
#endif

int main () { return 0; }
configure:7072: result: no, long long reliability test 1
configure:7310: error: could not find a working compiler, see config.log
for detail
"""

C23 removes unprototyped functions, which explains the error.

thanks,
sam