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
 
@@ -65,20 +66,23 @@ def _run_vivado(build_name, vivado_path, source, ver=None):
     else:
         build_script_contents = "# Autogenerated by Migen\nset -e\n"
 
-        # For backwards compatibility with ISE paths, also
-        # look for a version in a subdirectory named "Vivado"
-        # under the current directory.
-        paths_to_try = [vivado_path, os.path.join(vivado_path, "Vivado")]
-        for p in paths_to_try:
-            try:
-                settings = common.settings(p, ver)
-            except OSError:
-                continue
-            break
-        else:
-            raise OSError("Unable to locate Vivado directory or settings.")
+        # 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.
+            paths_to_try = [vivado_path, os.path.join(vivado_path, "Vivado")]
+            for p in paths_to_try:
+                try:
+                    settings = common.settings(p, ver)
+                except OSError:
+                    continue
+                break
+            else:
+                raise OSError("Unable to locate Vivado directory or settings.")
+            build_script_contents += "source " + settings + "\n"
 
-        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)