Allow building without SQLite3

[whitslack]

Summary of proposed changes:

• Introduce SQLITE3_CFLAGS and SQLITE3_LDLIBS config variables, whose values are automagically populated by calling out to pkg-config. If pkg-config is not found, then the new variables are defaulted as SQLITE3_CFLAGS= and SQLITE3_LDLIBS=-lsqlite3, which allows configure to continue working as before on systems lacking pkg-config.
  • When running configure, allow the user to override the name of pkg-config by setting the PKG_CONFIG environment variable. If unset, the default is pkg-config, which means to use the system default (i.e., in $PATH) copy of pkg-config. Cross-compiling users would set the PKG_CONFIG environment variable to the path of a pkg-config binary that provides information about packages built for the target system.
• Pass $SQLITE3_CFLAGS to Configurator, and add it to CFLAGS in Makefile.
• Pass $SQLITE3_LDLIBS in place of -lsqlite3, both in Makefile and in the Configurator script.
• Tweak a few source files so that they compile successfully when HAVE_SQLITE3=0.
• Omit devtools/checkchannels from build when HAVE_SQLITE3=0, as that tool requires SQLite3.

[rustyrussell]

Ack 38cd1a8

Nice work!

[cdecker]

ACK 38cd1a8