waf: toolchain: override find_g{xx,cc}() instead of wrapping configure()

That's more correct than setting Waf environment variables:

 (1) We don't need to worry about differences between OSes - the previous
 implementation was actually broken for Windows because the program names in
 the environment variables were missing the ".exe" extension.

 (2) That looks better than looping over possible compiler names and running
 the configuration transactionally multiple times.

Tools/ardupilotwaf/toolchain.py

@@ -20,30 +20,23 @@ from waflib.Tools import clang, clangxx, gcc, gxx
 import os
 import re
 
-def _set_toolchain_prefix_wrapper(tool_module, var, compiler_names):
-    original_configure = tool_module.configure
-    def new_configure(cfg):
-        if cfg.env.TOOLCHAIN == 'native':
-            original_configure(cfg)
-            return
+@conf
+def find_gxx(conf):
+    names = ['g++', 'c++']
+    if conf.env.TOOLCHAIN != 'native':
+        names = ['%s-%s' % (conf.env.TOOLCHAIN, n) for n in names]
+    cxx = conf.find_program(names, var='CXX')
+    conf.get_cc_version(cxx, gcc=True)
+    conf.env.CXX_NAME = 'gcc'
 
-        last_exception = None
-        for name in compiler_names:
-            cfg.env.stash()
-            try:
-                cfg.env[var] = '%s-%s' % (cfg.env.TOOLCHAIN, name)
-                original_configure(cfg)
-            except Errors.ConfigurationError as e:
-                cfg.env.revert()
-                last_exception = e
-            else:
-                cfg.env.commit()
-                return
-        raise last_exception
-    tool_module.configure = new_configure
-
-_set_toolchain_prefix_wrapper(gxx, 'CXX', ('g++', 'c++'))
-_set_toolchain_prefix_wrapper(gcc, 'CC', ('gcc', 'cc'))
+@conf
+def find_gcc(conf):
+    names = ['gcc', 'cc']
+    if conf.env.TOOLCHAIN != 'native':
+        names = ['%s-%s' % (conf.env.TOOLCHAIN, n) for n in names]
+    cc = conf.find_program(names, var='CC')
+    conf.get_cc_version(cc, gcc=True)
+    conf.env.CC_NAME = 'gcc'
 
 def _clang_cross_support(cfg):
     if _clang_cross_support.called: