Recently I was playing with custom builds of libsqlite3 and neither
-lsqlite3 without other options find the library, not the compilation at
all works because libsqlite3 headers cannot be found.

There is established way to make this all working: by using pkg-config
builder can query it for a "package" cflags or libs e.g. this way:

	$ pkg-config --cflags sqlite3
	-I/home/kirr/local/sqlite/include

	$ pkg-config --libs sqlite3
	-L/home/kirr/local/sqlite/lib -lsqlite3

and use so-provided flags for the build.

And if the build is not custom pkg-config will just report cflags/libs
for a system package:

	$ pkg-config --cflags sqlite3

	$ pkg-config --libs sqlite3
	-lsqlite3

Since CGo provides support for pkg-config out of the box [1] we can use
it to get proper libsqlite3 CFLAGS/LDFLAGS.

I guess FreeBSD flags (added in e90ca0e4) becomes also not needed,
because by default pkg-config searches for package descriptions in
/usr/local/{lib,share}/pkgconfig too, so I'm switching that to
pkg-config too.

We leave explicit `-lsqlite3` to be there for non-(linux or freebsd) to
support Windows and Mac where pkg-config is probably not available by
default.

[1] https://golang.org/cmd/cgo/#Shdr-Using_cgo_with_the_go_command

open: Fix error message

By accident I was trying to open a non-existing database file and got
the following message:

	unable to open database file (unable to open database file)

which was not helpful.

By convention the error returned by a function should contain the
context passed to that function - the filename in Open case, the
operation performed and the error description. So

- teach OpenError to print filename of the opened database and the
  operation.
- don't duplicate the code in braces if we have the error message
  with the same text because they are often redundant.
- if we have only error code but not a message, still we have to include
  the filename and operation in the printed context.
- in the Open itself: always return OpenError on handling errors from
  sqlite3_open_v2 even if db == nil - instead of returning only Errno
  without any context.

Now the error in my case is:

	notfound.db: unable to open database file

( @gwenn requested to omit the ": open" (operation) probable because it
  duplicates error detail a bit )

/helped-by @gwenn

driver: Don't waste resources setting/being-notified up in progress handler if context cannot be canceled

driver: Don't waste resources setting/being-notified up in progress handler if context cannot be canceled

Please see https://github.com/mattn/go-sqlite3/pull/530 for similar
change. Here it is not additional goroutine but a callback setup which
will be called ever 100 ops. We can avoid that if we know that ctx
cannot be canceled.

Caveat: it will not be possible to retry on SQLITE_BUSY.

Add working cgo defaults for FreeBSD (#7)

To be fixed: cgocheck & panic

Still missing: StmtExecContext and StmtQueryContext

To be continued

It does not seem to be specify but it fails at runtime.