build: reduce autoconf dependency to 2.67 (from 2.69)

Signed-off-by: Evan Nemerson <evan@coeus-group.com>
Fix warnings and errors from clang.

Signed-off-by: Evan Nemerson <evan@coeus-group.com>
Fix warnings from icc.

Signed-off-by: Evan Nemerson <evan@coeus-group.com>
Fix a warning from tcc.

Squash is still not able to build with tcc, apparently due to a problem
with libtool passing it an option it doesn't understand when building
the plugins.

Signed-off-by: Evan Nemerson <evan@coeus-group.com>