Brace for impact
Today I was looking around for a bug in autoconf, and I noticed one interesting bit out of the NEWS file of the current git version:
Present But Cannot Be Compiled: Autoconf will now proceed with the compiler’s result if a header is present but cannot be compiled. The warning is still printed, and you should really fix it by providing a fourth parameter to AC_CHECK_HEADER/AC_CHECK_HEADERS.
This is a tremendously useful thing to know before autoconf 2.64 is released, which is hopefully not too soon. The reason for this is that finally, after years of having that as a warning, to the point that some projects even ignored it altogether, the new autoconf will start ignoring header files that cannot be compiled, for whatever reason. This is useful since it ensures that headers are not detected that lacks proper dependencies. Unfortunately this also means that any software that currently relies on header files found without compilation will change behaviour.