dev-libs/gmp fails to detect double format

[sjnewbury]

During configure gmp detects the format of various data types to enable optimal conversions. When LTO is enabled it detects double type "unknown" and issues a warning about resulting slow performance.

It otherwise builds correctly, so this could have been easily missed. The detection code needs to be improved to cover the LTO case or LTO needs to be disabled to have a performant libgmp.

[InBetweenNames]

Oh wow, looking at this right now. They are basically scanning the produced .o files to determine the floating point format.

[InBetweenNames]

There are a few other problems in the configure script as well, although stripping CFLAGS from that test in particular seems to fix it. Notably, it does preserve the GCC target machine option.

[InBetweenNames]

Interesting link: https://www.gnu.org/software/autoconf/manual/autoconf-2.63/html_node/Cache-Files.html

I'm wondering if it might be possible to use this or something like it to "force" the test to resolve a normal way, for configure scripts in general. There was another issue on here somewhat related to this, but I'm having trouble finding it.