GMP is needed for checking digital signatures on the clam virus definitions database so if you want clamav on your system you will need this. For use on an HLFS system this package needs some extra effort. First get a patch from gentoo hardened: http://ftp.riken.go.jp/Linux/gentoo/dev-libs/gmp/files/gmp-4.1.4-noexecstack.patch
patch -Np1 -i ../gmp-4.1.4-noexecstack.patch
Compile the package with:
env CC="gcc -fno-stack-protector -fno-pic -fno-pie -nopie" \ ./configure --prefix=/usr --enable-static=no --enable-shared=yes -with-pic && make && make check
If the test wont give you any messages containing "cannot enable executable stack as shared object requires" you can continue with a make install according the BLFS book.