net-proxy/squid: fix cross-compiling

Current versions of squid have a run test for gnu atomics w/no
cross-compile fallback causing builds to fail.  Upstream has
rewritten this completely, so until we update to a newer version,
force the cache variable when cross-compiling.  Forcing it to no
might not be optimal in all cases, but it's better than nothing.

We have been turning on large-file-system support for a while now,
but the configure test for that probes the build system for flags
to compile with (via `getconf ...`).  These flags don't work for
the cross-compile target, and in reality are rarely necessary.  By
selecting the default, we always go the standard POSIX route with
the -D_FILE_OFFSET_BITS=64 flag.  If a target doesn't respect that
flag, then it might fail to build, but we can wait for anyone to
complain.

net-proxy/squid/squid-3.5.27.ebuild

@@ -158,6 +158,10 @@ src_configure() {
 	export BUILDCXXFLAGS=${BUILD_CXXFLAGS}
 	tc-export CC AR
 
+	# Should be able to drop this workaround with newer versions.
+	# https://bugs.squid-cache.org/show_bug.cgi?id=4224
+	tc-is-cross-compiler && export squid_cv_gnu_atomics=no
+
 	econf \
 		--sysconfdir=/etc/squid \
 		--libexecdir=/usr/libexec/squid \
@@ -182,6 +186,7 @@ src_configure() {
 		--enable-icmp \
 		--enable-follow-x-forwarded-for \
 		--with-large-files \
+		--with-build-environment=default \
 		--disable-strict-error-checking \
 		--disable-arch-native \
 		--with-ltdl-includedir=/usr/include \