Include config.h in source files.

Else, we don't get any definitions required for enforcing standard compliance.
Fixed (pedantic) void-pointer to function-pointer cast warnings.
Removed semicolon from SDB_PLUGIN_MAGIC definition.

Rather, the semicolon is added when using the define.
configure: Include _BSD_SOURCE in standards related defines.

Else, GNU libc does not define strcasecmp() even though it conforms to
POSIX.1-2001.
configure: Request X/Open 7 (XSI) compliance.

This is required for strndup().
configure: Disable -Wsign-conversion.

It may cause problems in expanded macros from libc.
configure: Make "standards" and "hardening" flags part of STRICT_CFLAGS.

Most importantly, don't assign them to CFLAGS which might be overwritten when
running make, thus leaving out the flags which were originally requested to be
used.
configure: Don't use -pedantic.

Else, GCC will complain about "ISO C forbids conversion of object pointer to
function pointer type", even for casting function points to or from void
pointers. This is allowed by POSIX, though, and required, for example, for
dlsym() to work.