configure.ac: Print $host and $build.

In the configure summary at the end also print the architecture we're
building on ($build) and the target host we build for ($host). The two are
not necessarily the same, e.g. in the case of cross-compiles.

configure.ac

@@ -460,6 +460,8 @@ echo
 echo "  - Package version (major.minor.micro):    $SR_PACKAGE_VERSION"
 echo "  - Library version (current:revision:age): $SR_LIB_VERSION"
 echo "  - Prefix: $prefix"
+echo "  - Building on: $build"
+echo "  - Building for: $host"
 echo
 echo "Detected libraries:"
 echo