package/libsigc: bump to version 2.10.6

package/libsigc: bump to version 2.10.6

- Switch to meson-package as configure is not shipped in the official
  tarball anymore
- Drop host-m4 dependency (only needed in maintainer mode)
- Disable examples and XML validation (enabled by default)
- Drop LIBSIGC_INSTALL_TARGET_FIXUP as documentation is disabled by
  default
- Update web page in Config.in
- Update indentation in hash file (two spaces)

https://github.com/libsigcplusplus/libsigcplusplus/blob/2.10.6/NEWS

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>