Use stash/revert/commit pattern for transactional configuration

Apply that pattern when relevant. That allows correct behavior if tools are
loaded inside a transaction.

waflib/Tools/compiler_c.py

@@ -81,7 +81,9 @@ def configure(conf):
 			if conf.env['CC']:
 				conf.end_msg(conf.env.get_flat('CC'))
 				conf.env['COMPILER_CC'] = compiler
+				conf.env.commit()
 				break
+			conf.env.revert()
 			conf.end_msg(False)
 	else:
 		conf.fatal('could not configure a C compiler!')

waflib/Tools/compiler_cxx.py

@@ -82,7 +82,9 @@ def configure(conf):
 			if conf.env['CXX']:
 				conf.end_msg(conf.env.get_flat('CXX'))
 				conf.env['COMPILER_CXX'] = compiler
+				conf.env.commit()
 				break
+			conf.env.revert()
 			conf.end_msg(False)
 	else:
 		conf.fatal('could not configure a C++ compiler!')

waflib/Tools/compiler_d.py

@@ -58,7 +58,9 @@ def configure(conf):
 			if conf.env.D:
 				conf.end_msg(conf.env.get_flat('D'))
 				conf.env['COMPILER_D'] = compiler
+				conf.env.commit()
 				break
+			conf.env.revert()
 			conf.end_msg(False)
 	else:
 		conf.fatal('could not configure a D compiler!')

waflib/Tools/compiler_fc.py

@@ -44,7 +44,9 @@ def configure(conf):
 			if conf.env['FC']:
 				conf.end_msg(conf.env.get_flat('FC'))
 				conf.env.COMPILER_FORTRAN = compiler
+				conf.env.commit()
 				break
+			conf.env.revert()
 			conf.end_msg(False)
 	else:
 		conf.fatal('could not configure a Fortran compiler!')

waflib/extras/boost.py

@@ -410,12 +410,14 @@ def check_boost(self, *k, **kw):
 				self.env["CXXFLAGS_%s" % var] += cxxflags
 				try:
 					try_link()
-					self.end_msg("ok: winning cxxflags combination: %s" % (self.env["CXXFLAGS_%s" % var]))
-					exc = None
-					break
 				except Errors.ConfigurationError as e:
 					self.env.revert()
 					exc = e
+				else:
+					self.end_msg("ok: winning cxxflags combination: %s" % (self.env["CXXFLAGS_%s" % var]))
+					exc = None
+					self.env.commit()
+					break
 
 			if exc is not None:
 				self.end_msg("Could not auto-detect boost linking flags combination, you may report it to boost.py author", ex=exc)