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Sébastien, while this gives expected results on the AIX box here, it still has one subtle problem:
AC_TRY_LINK may fail due to missing declaration in <sys/file.h> _or_ due to missing implementation in libc.
The subsequent AC_CHECK_LIB won't fail when the implementation is in libbsd but the declaration is missing - this is the case on not-so-recent AIX5.3 (and the reason python worked there before).
Another minor nit is configure's output when libbsd is needed:
> checking for flock... checking for flock in -lbsd... yes
> yes
Anyway - I've hacked up another variant (with recent autoconf manual at hand), which really should cover all known situations now (additionally using cache-checks):
AC_CACHE_CHECK([for flock declaration], [ac_cv_flock_decl],
 [AC_COMPILE_IFELSE(
 [AC_LANG_PROGRAM(
 [#include <sys/file.h>],
 [void* p = flock]
 )],
 [ac_cv_flock_decl=yes],
 [ac_cv_flock_decl=no]
 )
])
if test "x${ac_cv_flock_decl}" = xyes; then
 AC_CHECK_FUNCS(flock,,
 AC_CHECK_LIB(bsd,flock,
 [AC_DEFINE(HAVE_FLOCK)
 AC_DEFINE(FLOCK_NEEDS_LIBBSD, 1, Define if flock needs to be linked with bsd library.)
 ])
 )
fi
Thank you anyway!