Enable check-abi by default

[Alfred M. Szmidt]

   > -	      AC_HELP_STRING([--enable-check-abi],
   > -			     [do "make check-abi" in "make check" (no/warn/yes)
   > +	      AC_HELP_STRING([--disable-check-abi],
   > +			     [do not "make check-abi" in "make check" (no/warn/yes)

   The help string makes no sense any more.

`Disable ABI tests when running the test-suite.'