jpeg: more fixes for clang 16

[chrstphrchvz]

Includes the change from #93

Note that the CI workflow currently only builds Perl/Tk against system libjpeg (the configure step output contains Using system's -ljpeg rather than Building jpeg/libjpeg.a) and so will not test the changes proposed to this configure script.

[stemd]

This pull is also working with clang 21, and if for any reason you don't have any other compiler but clang, it solves errors like:

Building jpeg/libjpeg.a
checking for gcc... cc
checking whether the C compiler (cc -fno-common -DPERL_DARWIN -DNO_THREAD_SAFE_QUERYLOCALE -DNO_POSIX_2008_LOCALE -DHAS_BROKEN_LANGINFO_CODESET -DNO_LOCALE_COLLATE -fno-strict-aliasing -pipe -fstack-protector-strong   -fno-common -DPERL_DARWIN -DNO_THREAD_SAFE_QUERYLOCALE -DNO_POSIX_2008_LOCALE -DHAS_BROKEN_LANGINFO_CODESET -DNO_LOCALE_COLLATE -fno-strict-aliasing -pipe -fstack-protector-strong  -fstack-protector-strong) works... no
configure: error: installation or configuration problem: C compiler cannot create executables.

I applied this pull and then built Tk without any other problem.

int main() instead of main() should not break any compiler, same is valid for adding #include <stdio.h> or #ifdef block.