Allow to disable 'register custom scheme' functions using a macro.

Adding "DEFINES += TDESKTOP_DISABLE_REGISTER_CUSTOM_SCHEME" to Telegram.pro would disable these functions. This can be useful when the application is distributed using an installer or a package manager that can apply these
settings system-globaly.

Signed-off-by: Eduardo Sánchez Muñoz <eduardosanchezmunoz@gmail.com> (github: eduardosm)

Telegram/SourceFiles/pspecific_linux.cpp

@@ -1165,6 +1165,7 @@ namespace {
 }
 
 void psRegisterCustomScheme() {
+    #ifndef TDESKTOP_DISABLE_REGISTER_CUSTOM_SCHEME
     QString home(_psHomeDir());
     if (home.isEmpty()) return;
 
@@ -1249,6 +1250,7 @@ void psRegisterCustomScheme() {
             LOG(("App Error: Could not open '%1' for write").arg(file));
         }
     }
+    #endif
 }
 
 void psNewVersion() {