BLD: try linking a file if compile long double repr detection fails

When compiler uses link time optimization it will not emit the binary
representation of the long double during linking. This prevents building
numpy with -flto.
To avoid this try linking the object when the compile detection failed.
With this change numpy can be built with link time optimization with 4 worker
processes using following command:

CC='gcc -fno-fat-lto-objects -flto=4 -fuse-linker-plugin -O3' \
LDSHARED='gcc -fno-fat-lto-objects -flto=4 -fuse-linker-plugin -shared
-O3' AR=gcc-ar \
python setup.py build_ext

See also:
https://www.mail-archive.com/numpy-discussion@scipy.org/msg45535.html

Should also work with icc's -ipo switch (probably closes gh-4992)