how to add a library if it exists

[Andrew Makhorin]

Hi,

My question is probably very simple to autoconf experts, however,
I could not find the answer in the autoconf documentation.

I need to include a check in configure.am to see if libgmp.a (the GNU
bignum library) exists. And if it exists, the configure script has to
do the following:

1) change one line in some header file (by sed, for example);

2) add the option '-lgmp' in makefiles, where binaries are compiled.

Thank you,

Andrew Makhorin