More edits

src/doc/en/installation/source.rst

@@ -21,6 +21,7 @@ better suit you. The instructions support most of the platforms on a personal
 computer. However, if you encounter problems or you plan to install Sage on an
 unusual environment, you may find more information in the sections below.
 
+
 .. _section-prereqs:
 
 Prerequisites
@@ -254,7 +255,6 @@ Some additional optional packages are taken care of by:
 
 .. literalinclude:: homebrew-optional.txt
 
-
 WSL prerequisites
 ^^^^^^^^^^^^^^^^^
 
@@ -800,6 +800,7 @@ On slow hardware building and testing Sage can even take several days!
 
 Interfacing other software
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 Optional. Check the interfaces to any other software that you have available.
 Note that each interface calls its corresponding program by a particular
 name: `Mathematica <https://www.wolfram.com/mathematica/>`_ is invoked by
@@ -951,10 +952,11 @@ how it is built:
   unpacking the source tarfile again, the only difference being that the
   :file:`.git` directory is preserved, so git branches are not deleted.
 
+
 .. _section_envvar:
 
-Environment variables
----------------------
+Build-time environment variables
+--------------------------------
 
 Sage uses several environment variables to control its build process.
 Most users won't need to set any of these: the build process just works on many
@@ -964,7 +966,6 @@ speed up the process.)
 Building Sage involves building many packages, each of which has its own
 compilation instructions.
 
-
 Standard environment controlling the build process
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -1334,7 +1335,6 @@ Environment variables controlling the documentation build
 
     before opening the Sage documentation webpage.
 
-
 Environment variables dealing with specific Sage packages
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -1370,7 +1370,6 @@ Environment variables dealing with specific Sage packages
   The value of this variable is passed as an
   argument to the ``$MAKE`` command when compiling PARI.
 
-
 Environment variables dealing with doctesting
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -1397,7 +1396,6 @@ Environment variables dealing with doctesting
   for more information on the effects of these flags (and therefore
   these variables).
 
-
 Environment variables set within Sage environments
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -1607,6 +1605,7 @@ Now let's build Sage, starting with the step::
 Refer to the section :ref:`build-from-source-step-by-step` for the following
 steps.
 
+
 Relocation
 ----------
 
@@ -1619,6 +1618,7 @@ user and then as root you can change permissions. See the `Installation
 Guide <https://doc.sagemath.org/html/en/installation/source.html#installation-in-a-multiuser-environment>`__
 for further information.
 
+
 Redistribution
 --------------
 
@@ -1639,6 +1639,7 @@ do.
 
    The result is placed in the directory ``dist/``.
 
+
 Build system
 ------------
 
@@ -1693,6 +1694,7 @@ of the source tree goes something like this:
    dependencies first (and their dependencies, recursively); the actual
    package installation is performed with the ``sage-spkg`` program
 
+
 Directory layout
 ----------------