Search debugger first in selected CBS toolchain. (#1033)

For Core Build System local debug target. If there is no absolute path
set in the Debugger tab of the launch configuration, try to find the
debugger first in the selected toolchain. If the debugger is not found
in the toolchain, let GdbLaunch search in PATH.
If an absolute path is set, GdbLaunch will use that.

Fixes #1008

dsf-gdb/org.eclipse.cdt.dsf.gdb/src/org/eclipse/cdt/dsf/gdb/internal/launching/CoreBuildLocalDebugLaunchDelegate.java

@@ -10,18 +10,22 @@
  *******************************************************************************/
 package org.eclipse.cdt.dsf.gdb.internal.launching;
 
+import java.nio.file.Path;
+import java.nio.file.Paths;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Properties;
 import java.util.concurrent.ExecutionException;
 
 import org.eclipse.cdt.core.build.ICBuildConfiguration;
+import org.eclipse.cdt.core.build.IToolChain;
 import org.eclipse.cdt.debug.core.launch.CoreBuildLaunchConfigDelegate;
 import org.eclipse.cdt.dsf.concurrent.DataRequestMonitor;
 import org.eclipse.cdt.dsf.concurrent.ImmediateExecutor;
 import org.eclipse.cdt.dsf.concurrent.Query;
 import org.eclipse.cdt.dsf.concurrent.RequestMonitorWithProgress;
 import org.eclipse.cdt.dsf.concurrent.Sequence;
+import org.eclipse.cdt.dsf.gdb.IGDBLaunchConfigurationConstants;
 import org.eclipse.cdt.dsf.gdb.internal.GdbPlugin;
 import org.eclipse.cdt.dsf.gdb.internal.Messages;
 import org.eclipse.cdt.dsf.gdb.launching.GdbLaunch;
@@ -78,6 +82,18 @@ public void launch(ILaunchConfiguration configuration, String mode, ILaunch laun
 		envProps.putAll(buildEnv);
 		gdbLaunch.setInitialEnvironment(envProps);
 
+		String debugger = configuration.getAttribute(IGDBLaunchConfigurationConstants.ATTR_DEBUG_NAME,
+				IGDBLaunchConfigurationConstants.DEBUGGER_DEBUG_NAME_DEFAULT);
+		Path debuggerPath = Paths.get(debugger);
+		// Try to find the debugger in the toolchain if the name is not absolute.
+		if (!debuggerPath.isAbsolute()) {
+			IToolChain toolChain = buildConfig.getToolChain();
+			Path gdbPath = toolChain.getCommandPath(debuggerPath);
+			if (gdbPath != null) {
+				gdbLaunch.setGDBPath(gdbPath.toString());
+			}
+			// When not found, GdbLaunch will search the debugger in PATH.
+		}
 		String gdbVersion = gdbLaunch.getGDBVersion();
 
 		gdbLaunch.setProgramPath(getProgramPath(configuration, buildConfig));