Core/Common: Add a generic config helper to access built-in overwriteable paths.

* Adds CMAKE_COMMAND and CMAKE_BINARY_DIR to revision_data.h
* Move the source and mysql exe path handling out of the DBUpdater.
* Make some Config methods const for correctness.
* Remove C & CXX flags from revision_data.h
 (was unused and didn't capture all cxx vars)
* Reorder the link order to prevent `ld` from ignoring the file
* Ref #15671

src/common/Configuration/BuiltInConfig.cpp

@@ -0,0 +1,52 @@
+/*
+ * Copyright (C) 2008-2016 TrinityCore <http://www.trinitycore.org/>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "BuiltInConfig.h"
+#include "Config.h"
+#include "GitRevision.h"
+
+template<typename Fn>
+static std::string GetStringWithDefaultValueFromFunction(
+    std::string const& key, Fn getter)
+{
+    std::string const value = sConfigMgr->GetStringDefault(key, "");
+    return value.empty() ? getter() : value;
+}
+
+std::string BuiltInConfig::GetCMakeCommand()
+{
+    return GetStringWithDefaultValueFromFunction(
+        "CMakeCommand", GitRevision::GetCMakeCommand);
+}
+
+std::string BuiltInConfig::GetBuildDirectory()
+{
+    return GetStringWithDefaultValueFromFunction(
+        "BuildDirectory", GitRevision::GetBuildDirectory);
+}
+
+std::string BuiltInConfig::GetSourceDirectory()
+{
+    return GetStringWithDefaultValueFromFunction(
+        "SourceDirectory", GitRevision::GetSourceDirectory);
+}
+
+std::string BuiltInConfig::GetMySQLExecutable()
+{
+    return GetStringWithDefaultValueFromFunction(
+        "MySQLExecutable", GitRevision::GetMySQLExecutable);
+}