Merge pull request #2018 from sriramsk1999/master

Included a useful CMAKE flag.

README.md

@@ -164,6 +164,7 @@ Options are specified with the -D flag.  The allowed options include:
     ARMADILLO_LIBRARY=(/path/to/armadillo/libarmadillo.so): Armadillo library
     BUILD_CLI_EXECUTABLES=(ON/OFF): whether or not to build command-line programs
     BUILD_PYTHON_BINDINGS=(ON/OFF): whether or not to build Python bindings
+    PYTHON_EXECUTABLE=(/path/to/python_version): Path to specific Python executable
     BUILD_TESTS=(ON/OFF): whether or not to build tests
     BUILD_SHARED_LIBS=(ON/OFF): compile shared libraries as opposed to
        static libraries

doc/guide/build.hpp

@@ -157,6 +157,7 @@ The full list of options mlpack allows:
  - DOWNLOAD_STB_IMAGE=(ON/OFF): If STB is not found, download it (default ON)
  - BUILD_WITH_COVERAGE=(ON/OFF): Build with support for code coverage tools
       (gcc only) (default OFF)
+ - PYTHON_EXECUTABLE=(/path/to/python_version): Path to specific Python executable     
  - BUILD_MARKDOWN_BINDINGS=(ON/OFF): Build Markdown bindings for website
        documentation (default OFF)
  - MATHJAX=(ON/OFF): use MathJax for generated Doxygen documentation (default