[build] Note default=MKL; cosmetic fix (#3257)

Author: nshmyrev

src/configure

@@ -82,7 +82,7 @@ Configuration options:
  --static-fst Build with static OpenFst libraries [default=no]
  --fst-root=DIR OpenFst root directory [default=../tools/openfst/]
  --fst-version=STR OpenFst version string
- --mathlib=LIB Math library [default=ATLAS]
+ --mathlib=LIB Math library [default=MKL]
  Supported libraries: ATLAS, MKL, CLAPACK, OPENBLAS.
  --static-math Build with static math libraries [default=no]
  --threaded-math Build with multi-threaded math libraries [default=no]