Consider <exe loc or cwd>/../ as a candidate for data dir autodetection

Makes it so <exe location>/../ (non-Windows) or <current working
dir>/../ (Windows) are also considered possible data directories if they
contain a data/_main.cfg file.

This is mostly intended to help people running cmake builds so they
don't have to type out .. in the Wesnoth command line every time or use
a hard link sitting on the data dir.

Notice that scons people already benefited from the simpler version of
this autodetection, which only considered <exe loc or cwd>/./ as a
candidate. This has been the case since version 1.5.4 (more
specifically, commit 43431bc). Because
of this, most of the code for this feature was already in place and I
only rearranged it to add some helpful annotations.

changelog

@@ -227,6 +227,9 @@ Version 1.13.0-dev:
    * Fix an inefficient implementation of unit::invisible, in an effort to address slow performance problems:
      http://forums.wesnoth.org/viewtopic.php?f=4&t=12139&start=180#p569931
      (Also see gfgtdf's commits trying to optimize the minimap loop)
+   * Made it so <exe location>/../ (non-Windows) or <current working dir>/../
+     (Windows) are also considered possible data directories if they contain a
+     data/_main.cfg file, intended to help with cmake builds.
 
 Version 1.11.11:
  * Add-ons server:

src/wesnoth.cpp

@@ -791,11 +791,28 @@ int main(int argc, char** argv)
 		const time_t t = time(NULL);
 		std::cerr << "Started on " << ctime(&t) << "\n";
 
-		const std::string exe_dir = get_exe_dir();
-		if(!exe_dir.empty() && file_exists(exe_dir + "/data/_main.cfg")) {
-			std::cerr << "Automatically found a possible data directory at "
-			          << exe_dir << '\n';
-			game_config::path = exe_dir;
+		const std::string& exe_dir = get_exe_dir();
+		if(!exe_dir.empty()) {
+			// Try to autodetect the location of the game data dir. Note that
+			// the root of the source tree currently doubles as the data dir.
+			std::string auto_dir;
+
+			// scons leaves the resulting binaries at the root of the source
+			// tree by default.
+			if(file_exists(exe_dir + "/data/_main.cfg")) {
+				auto_dir = exe_dir;
+			}
+			// cmake encourages creating a subdir at the root of the source
+			// tree for the build, and the resulting binaries are found in it.
+			else if(file_exists(exe_dir + "/../data/_main.cfg")) {
+				auto_dir = normalize_path(exe_dir + "/..");
+			}
+
+			if(!auto_dir.empty()) {
+				std::cerr << "Automatically found a possible data directory at "
+						  << auto_dir << '\n';
+				game_config::path = auto_dir;
+			}
 		}
 
 		const int res = do_gameloop(argc,argv);