nss: prevent NSS from crashing on client auth hook failure

Although it is not explicitly stated in the documentation, NSS uses
*pRetCert and *pRetKey even if the client authentication hook returns
a failure.  Namely, if we destroy *pRetCert without clearing *pRetCert
afterwards, NSS destroys the certificate once again, which causes a
double free.

Reported by: Bob Relyea
build: prevent global LIBS from influencing src and lib build targets

Currently, LIBS is already used through other macros.
configure: add minimal sanity check on user provided LIBS and LDFLAGS
multi: fix re-sending request on early connection close

This handling already works with the easy-interface code. When a request
is sent on a re-used connection that gets closed by the server at the
same time as the request is sent, the situation may occur so that we can
send the request and we discover the broken connection as a RECV_ERROR
in the PERFORM state and then the request needs to be retried on a fresh
connection. Test 64 broke with 'multi-always-internally'.
build: explain current role of LIBS in our Makefile.am files

BLANK_AT_MAKETIME may be used in our Makefile.am files to blank
LIBS variable used in generated makefile at makefile processing
time. Doing this functionally prevents LIBS from being used for
all link targets in given makefile.
examples: fix compilation issues
examples: fix compilation issues - commit 23f8dca6fb follow-up
examples: fix compilation issues - commit 7332a7cafb follow-up
libtest: fix some compiler warnings
build: fix compilation with CURL_DISABLE_CRYPTO_AUTH flag
Merge pull request #52 from isn-/master

small compilation fix
runtests and friends: Do not add undefined values to @INC

On FreeBSD this fixes the warning:
Use of uninitialized value $p in string eq at /usr/local/lib/perl5/5.14.2/BSDPAN/BSDPAN.pm line 36.
Introducing a new persistent connection caching system using "bundles".

A bundle is a list of all persistent connections to the same host.
The connection cache consists of a hash of bundles, with the
hostname as the key.
The benefits may not be obvious, but they are two:

1) Faster search for connections to reuse, since the hash
   lookup only finds connections to the host in question.
2) It lays out the groundworks for an upcoming patch,
   which will introduce multiple HTTP pipelines.

This patch also removes the awkward list of "closure handles",
which were needed to send QUIT commands to the FTP server
when closing a connection.
Now we allocate a separate closure handle and use that
one to close all connections.

This has been tested in a live system for a few weeks, and of
course passes the test suite.
configure: fix cross pkg-config detection

When cross-compiling, CURL_CHECK_PKGCONFIG was checking for the cross
pkg-config using ${host}-pkg-config.

The gold standard for doing this correctly is pkg-config's own macro,
PKG_PROG_PKG_CONFIG.  However, on the assumption that you have a good
reason not to use that directly (reduced dependencies for maintainer
builds?), the behaviour of cURL's version should at least match.
PKG_PROG_PKG_CONFIG uses AC_PATH_TOOL, which ultimately ends up trying
${host_alias}-pkg-config; this is not quite the same as what cURL does,
and may differ because ${host} has been run through config.sub.  For
instance, when cross-building to the armhf architecture on Ubuntu,
${host_alias} is arm-linux-gnueabihf while ${host} is
arm-unknown-linux-gnueabihf.  This may also have been the cause of the
problem reported at http://curl.haxx.se/mail/lib-2012-04/0224.html.

AC_PATH_TOOL is significantly simpler than cURL's current code, and
dates back to well before the current minimum of Autoconf 2.57, so let's
use it instead.
examples/anyauthput.c: fix Tru64 compilation issue
conncache: fix enumerated type mixed with another type
build: add bundles and conncache files to other build systems
examples/simplessl.c: fix compiler warning
examples/externalsocket.c: fix SunPro compilation issue
examples/simplessl.c: fix compiler warning