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configure.in determines if -Werror-implicit-function-declaration is
available, and if so it enables that flag if --enable-developer is
specified. Since the configure tests themselves did not use that flag, it
was possible for a configure test to succeed, followed by a failed
compilation due to a facility being available but not having a proper
declaration in a header file. (This bit me with readahead().) This patch
ensures that if implicit function declarations will kill the build, the
feature being tested is deselected so the build will succeed.
The autoconf manual suggests using return instead of exit in configure
tests because the declaration for exit is often missing. We require this
now, since we error if prototypes are missing. See section 5.5.1 of
http://www.gnu.org/software/autoconf/manual/autoconf.html. This patch makes
these changes, because in fact, an external declaration for exit is missing
here (and likely elsewhere).
I've verified that the features selected (here) with the original
configure.in and the new one are the same except for, in my case,
readahead. I've also confirmed that the generated Makefile is identical.
These changes are not being applied to the 3.0.26 branch because it does not
exhibit the initial problem this patch is supposed to solve since it doesn't
attempt to use -Werror-implicit-function-declaration.
(This used to be commit 4d42720915b8f65842147171f689127e94d1b973)
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