Add support for breaking into debugger for Linux

Use Linux-specific /proc/$PID/status file to check whether we're being
debugged and a generic raise(SIGTRAP) to actually break into the debugger.

include/internal/catch_debugger.hpp

@@ -61,6 +61,33 @@
         }
     } // namespace Catch
 
+#elif defined(CATCH_PLATFORM_LINUX)
+    #include <fstream>
+    #include <string>
+
+    namespace Catch{
+        // The standard POSIX way of detecting a debugger is to attempt to
+        // ptrace() the process, but this needs to be done from a child and not
+        // this process itself to still allow attaching to this process later
+        // if wanted, so is rather heavy. Under Linux we have the PID of the
+        // "debugger" (which doesn't need to be gdb, of course, it could also
+        // be strace, for example) in /proc/$PID/status, so just get it from
+        // there instead.
+        bool isDebuggerActive(){
+            std::ifstream in("/proc/self/status");
+            for( std::string line; std::getline(in, line); ) {
+                static const int PREFIX_LEN = 11;
+                if( line.compare(0, PREFIX_LEN, "TracerPid:\t") == 0 ) {
+                    // We're traced if the PID is not 0 and no other PID starts
+                    // with 0 digit, so it's enough to check for just a single
+                    // character.
+                    return line.length() > PREFIX_LEN && line[PREFIX_LEN] != '0';
+                }
+            }
+
+            return false;
+        }
+    } // namespace Catch
 #elif defined(_MSC_VER)
     extern "C" __declspec(dllimport) int __stdcall IsDebuggerPresent();
     namespace Catch {