Add seealso pointing to Buildsystem intro in all compiling pages

Many new contributors want to add information on -j multicore option for
SCons in "Compiling for X" pages as they seem to skip the intro, so let's
make it prominent.

development/compiling/compiling_for_android.rst

@@ -30,6 +30,9 @@ For compiling under Windows, Linux or macOS, the following is required:
 -  Gradle (will be downloaded and installed automatically if missing)
 -  JDK 6 or later (either OpenJDK or Oracle JDK) - JDK 9 & 10 do not work with current Gradle.
 
+.. seealso:: For a general overview of SCons usage for Godot, see
+             :ref:`doc_introduction_to_the_buildsystem`.
+
 Setting up the buildsystem
 --------------------------
 
@@ -50,18 +53,6 @@ To set those environment variables on Unix (e.g. Linux, macOS), use
 Where /path/to/android-sdk and /path/to/android-ndk is the path where Android SDK
 and Android NDK are placed on your PC.
 
-Toolchain
-~~~~~~~~~
-
-We usually try to keep the Godot Android build code up to date, but
-Google changes their toolchain versions often, so if compilation
-fails due to wrong toolchain version, go to your NDK directory and check
-the current number, then set the following environment variable:
-
-::
-
-    NDK_TARGET (by default set to "arm-linux-androideabi-4.9")
-
 Building the export templates
 -----------------------------
 

development/compiling/compiling_for_ios.rst

@@ -12,6 +12,9 @@ Requirements
    ``scons`` in a terminal when installed)
 -  Xcode with the iOS SDK and the command line tools.
 
+.. seealso:: For a general overview of SCons usage for Godot, see
+             :ref:`doc_introduction_to_the_buildsystem`.
+
 Compiling
 ---------
 

development/compiling/compiling_for_osx.rst

@@ -15,6 +15,9 @@ required:
 -  SCons build system
 -  Xcode (or the more lightweight Command Line Tools for Xcode)
 
+.. seealso:: For a general overview of SCons usage for Godot, see
+             :ref:`doc_introduction_to_the_buildsystem`.
+
 Compiling
 ---------
 

development/compiling/compiling_for_uwp.rst

@@ -17,6 +17,9 @@ Requirements
    ``ms_master`` (default) branch. Keep it in a path without spaces to
    avoid problems.
 
+.. seealso:: For a general overview of SCons usage for Godot, see
+             :ref:`doc_introduction_to_the_buildsystem`.
+
 Compiling
 ---------
 

development/compiling/compiling_for_web.rst

@@ -16,6 +16,9 @@ To compile export templates for the Web, the following is required:
 -  `Python 2.7+ or Python 3.5+ <https://www.python.org/>`__
 -  `SCons <https://www.scons.org>`__ build system
 
+.. seealso:: For a general overview of SCons usage for Godot, see
+             :ref:`doc_introduction_to_the_buildsystem`.
+
 Building export templates
 -------------------------
 

development/compiling/compiling_for_windows.rst

@@ -20,6 +20,9 @@ For compiling under Windows, the following is required:
    for parallel builds (which increase the build speed by a great factor).
 -  `SCons <https://www.scons.org>`__ build system.
 
+.. seealso:: For a general overview of SCons usage for Godot, see
+             :ref:`doc_introduction_to_the_buildsystem`.
+
 Setting up SCons
 ----------------
 

development/compiling/compiling_for_x11.rst

@@ -24,6 +24,9 @@ required:
 -  *Optional* - libudev (build with ``udev=yes``)
 -  *Optional* - yasm (for WebM SIMD optimizations)
 
+.. seealso:: For a general overview of SCons usage for Godot, see
+             :ref:`doc_introduction_to_the_buildsystem`.
+
 Distro-specific oneliners
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 +---------------+------------------------------------------------------------------------------------------------------------+

development/compiling/introduction_to_the_buildsystem.rst

@@ -39,6 +39,7 @@ build Godot yourself.
 
 Setup
 -----
+
 Please refer to the documentation for :ref:`doc_compiling_for_android`, :ref:`doc_compiling_for_ios`, :ref:`doc_compiling_for_osx`, :ref:`doc_compiling_for_uwp`, :ref:`doc_compiling_for_web`, :ref:`doc_compiling_for_windows` and :ref:`doc_compiling_for_x11`.
 
 Note that for **Windows/Visual Studio**, you need to use ``x86_x64 Cross Tools Command Prompt for VS 2017`` or similar, depending on your install, instead of the standard Windows command prompt to enter the commands below.