imagemagick: copy *-config files to staging dir

imagemagick: copy *-config files to staging dir

We need to copy MagickWand-config, which handles pkg-config data, to the
staging dir, so that dependent packages can use it.  php7-pecl-imagick,
for example, may otherwise use an eventual host-installed file, causing
compilation to fail.  There's also a MagickCore-config file that does
the same thing for the MagickCore library, so a *-config wilcard is used
to handle all cases.

Since configure is called with --prefix=/usr, use $(SED) to change
definition of prefix and exec_prefix lines in *-config from /usr to
$(STAGING_DIR)/usr.

Signed-off-by: Eneas U de Queiroz <redacted>