autogen.sh: Add a /bin/sh shebang.
autogen.sh: Use set -e to fail if any command fails

In preparation for expanding autogen.sh.
autogen.sh: Support running from outside the source directory

Initial steps towards supporting out-of-tree builds.
autogen.sh: Use long options to autoreconf, for self-documentation
autogen.sh: Stop passing --verbose to autoreconf

This quiets down the autotools build, making warnings much more visible.
configure.ac: Check for miniupnpc headers, not just -lminiupnpc

This protects against broken systems which have libminiupnpc.a or
libminiupnpc.so installed but don't have the miniupnpc headers.
src/Makefile.am: Simplify clean of leveldb
Merge pull request #3322

26d1b65 src/Makefile.am: Simplify clean of leveldb (Josh Triplett)
a26a367 configure.ac: Check for miniupnpc headers, not just -lminiupnpc (Josh Triplett)
82ccb05 autogen.sh: Stop passing --verbose to autoreconf (Josh Triplett)
e12dafd autogen.sh: Use long options to autoreconf, for self-documentation (Josh Triplett)
19b9add autogen.sh: Support running from outside the source directory (Josh Triplett)
97d285a autogen.sh: Use set -e to fail if any command fails (Josh Triplett)
f80b723 autogen.sh: Add a /bin/sh shebang. (Josh Triplett)