configure: allow the gas-preprocessor to be called with an absolute path.

Allow the configure tool to call the gas-preprocessor with an absolute
path when included with either the "--as" or with a separate "--gas"
option.  The patch is backward compatible and will not break any current
configuration setups.

The new behavior that comes with this option can be utilized in the
following ways:

Examples:

- Just as single option that includes both the gas-preprocessor.pl and
  cc/as at the same time:

  $ configure --as="/opt/app/build-tools/gas-preprocessor.pl \
                   /Applications/Xcode.app/.../Developer/usr/bin/gcc"

- Call gas-preprocessor with a separate option in conjunction with --as:

  $ configure --gas="/opt/app/build-tools/gas-preprocessor.pl" \
               --as="/Applications/Xcode.app/.../Developer/usr/bin/gcc"

- Address only the the gas-preprocessor and it will automatically fall
  back to as or cc whichever that's defined:
  $ configure --gas="/opt/app/build-tools/gas-preprocessor.pl"

- Or if no gas-preprocessor.pl is explicitly defined, it falls back to
  the old behaivor.