build/xilinx/vivado: only try Xilinx setup if vivado is not already in the path

Only affects the non-Windows code path.
Uses python distutils, already used elsewhere.

litex/build/xilinx/vivado.py

@@ -5,6 +5,7 @@ import os
 import subprocess
 import sys
 import math
+from distutils import spawn
 
 from migen.fhdl.structure import _Fragment
 
@@ -65,6 +66,9 @@ def _run_vivado(build_name, vivado_path, source, ver=None):
     else:
         build_script_contents = "# Autogenerated by Migen\nset -e\n"
 
+        # No reason to search for vivado if it's already in our $PATH
+        # https://stackoverflow.com/questions/377017/test-if-executable-exists-in-python
+        if not spawn.find_executable("vivado"):
             # For backwards compatibility with ISE paths, also
             # look for a version in a subdirectory named "Vivado"
             # under the current directory.
@@ -77,8 +81,8 @@ def _run_vivado(build_name, vivado_path, source, ver=None):
                 break
             else:
                 raise OSError("Unable to locate Vivado directory or settings.")
-
             build_script_contents += "source " + settings + "\n"
+
         build_script_contents += "vivado -mode batch -source " + build_name + ".tcl\n"
         build_script_file = "build_" + build_name + ".sh"
         tools.write_to_file(build_script_file, build_script_contents)