[SCons] Allow specifying non-absolute paths for 'python_cmd'

This allows e.g. 'python_cmd=python3' without needing to know what
directory that actually comes from.

.travis.yml

@@ -71,8 +71,8 @@ script: |
         echo "Skipping documentation upload from source other than Cantera/cantera:master"
       fi
   else
-      python3 `which scons` build -j2 VERBOSE=y python_package=full blas_lapack_libs=lapack,blas optimize=n coverage=y
-      python3 `which scons` test
+      scons build -j2 python_cmd=python3 VERBOSE=y python_package=full blas_lapack_libs=lapack,blas optimize=n coverage=y
+      scons test
   fi
 after_success: |
   if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then

SConstruct

@@ -343,7 +343,7 @@ config_options = [
         """Cantera needs to know where to find the Python interpreter. If
            PYTHON_CMD is not set, then the configuration process will use the
            same Python interpreter being used by SCons.""",
-        sys.executable),
+        sys.executable, PathVariable.PathAccept),
     PathVariable(
         'python_prefix',
         """Use this option if you want to install the Cantera Python package to