build: improve FORTIFY_SOURCE usage
build: improve FORTIFY_SOURCE usage

Based on this coreutils bug report, since coreutils copied what
libvirt had done:
https://lists.gnu.org/archive/html/bug-coreutils/2012-10/msg00121.html

* m4/virt-compile-warnings.m4 (LIBVIRT_COMPILE_WARNINGS): Allow
for user to pre-define _FORTIFY_SOURCE.

VenkatDatta/libvirt,bjzhang/libvirt,eskultety/libvirt,datto/libvirt,andreabolognani/libvirt,nertpinx/libvirt,trainstack/libvirt,shugaoye/libvirt,zippy2/libvirt,cbosdo/libvirt,trainstack/libvirt,nertpinx/libvirt,trainstack/libvirt,agx/libvirt,iam-TJ/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,novel/fbsd-libvirt,jardasgit/libvirt,zhlcindy/libvirt-1.1.4-maintain,cbosdo/libvirt,zippy2/libvirt,jfehlig/libvirt,jeckersb/libvirt,siboulet/libvirt-openvz,elmarco/libvirt,siboulet/libvirt-openvz,iam-TJ/libvirt,olafhering/libvirt,jeckersb/libvirt,libvirt/libvirt,novel/fbsd-libvirt,agx/libvirt,rlaager/libvirt,olafhering/libvirt,jardasgit/libvirt,siboulet/libvirt-openvz,crobinso/libvirt,fabianfreyer/libvirt,datto/libvirt,agx/libvirt,libvirt/libvirt,nertpinx/libvirt,andreabolognani/libvirt,datto/libvirt,nertpinx/libvirt,zhlcindy/libvirt-1.1.4-maintain,shugaoye/libvirt,agx/libvirt,rlaager/libvirt,fabianfreyer/libvirt,cbosdo/libvirt,jeckersb/libvirt,elmarco/libvirt,olafhering/libvirt,datto/libvirt,libvirt/libvirt,taget/libvirt,trainstack/libvirt,elmarco/libvirt,eskultety/libvirt,elmarco/libvirt,iam-TJ/libvirt,emaste/libvirt,emaste/libvirt,crobinso/libvirt,crobinso/libvirt,jfehlig/libvirt,taget/libvirt,rlaager/libvirt,eskultety/libvirt,eskultety/libvirt,siboulet/libvirt-openvz,zhlcindy/libvirt-1.1.4-maintain,zippy2/libvirt,fabianfreyer/libvirt,iam-TJ/libvirt,agx/libvirt,emaste/libvirt,novel/fbsd-libvirt,jeckersb/libvirt,datto/libvirt,jardasgit/libvirt,elmarco/libvirt,taget/libvirt,novel/fbsd-libvirt,emaste/libvirt,jfehlig/libvirt,bjzhang/libvirt,emaste/libvirt,rlaager/libvirt,trainstack/libvirt,andreabolognani/libvirt,iam-TJ/libvirt,novel/fbsd-libvirt,jfehlig/libvirt,iam-TJ/libvirt,cbosdo/libvirt,zippy2/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,emaste/libvirt,taget/libvirt,eskultety/libvirt,bjzhang/libvirt,fabianfreyer/libvirt,emaste/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,novel/fbsd-libvirt,zhlcindy/libvirt-1.1.4-maintain,VenkatDatta/libvirt,iam-TJ/libvirt,jeckersb/libvirt,siboulet/libvirt-openvz,novel/fbsd-libvirt,shugaoye/libvirt,andreabolognani/libvirt,olafhering/libvirt,libvirt/libvirt,zhlcindy/libvirt-1.1.4-maintain,shugaoye/libvirt,novel/fbsd-libvirt,cbosdo/libvirt,trainstack/libvirt,rlaager/libvirt,shugaoye/libvirt,andreabolognani/libvirt,jeckersb/libvirt,jardasgit/libvirt,bjzhang/libvirt,taget/libvirt,crobinso/libvirt,bjzhang/libvirt,novel/fbsd-libvirt,jeckersb/libvirt,trainstack/libvirt