Implicit int error when using with Clang

Nobody has claimed this yet.

When configuring using amdclang on OLCF machines, I get:

```
configure: flink.c:1:1: error: type specifier missing, defaults to 'int'; ISO C99 and later do not support implicit int [-Wimplicit-int]
main(){ FF(); return 0; }
^
int
flink.c:1:9: error: call to undeclared function 'f_fun'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
main(){ FF(); return 0; }
^
:1:12: note: expanded from here
#define FF f_fun
^
2 errors generated.
configure: error: giving up
```

which can be remedied by changing

`echo "main(){ FF(); return 0; }" > flink.c`

to

`echo "int main(){ int FF(); return 0; }" > flink.c`

in `configure` and `configure.ac`

Any reason not to?

Start by comparing the flink.c probe command in configure and configure.ac, then reproduce the configuration with amdclang on an OLCF machine if available. Verify that the updated probe completes without implicit-int or undeclared-function errors and that the generated configure behavior remains consistent.

Written by the indexing model from the issue text.