Trac #34889: Installation guide: On WSL, clear /mnt/c stuff from PATH

As seen in https://groups.google.com/g/sage-devel/c/1nYw7-6QSkQ, our
`configure` script fails to notice that some stuff in finds in PATH is
not suitable:
{{{
configure:7510: found /mnt/c/Strawberry/perl/bin/pkg-config
configure:7522: result: /mnt/c/Strawberry/perl/bin/pkg-config
configure:7547: checking pkg-config is at least version 0.29
...
configure:33252: checking for xml2-config
configure:33275: found /mnt/c/Strawberry/c/bin/xml2-config
configure:33288: result: /mnt/c/Strawberry/c/bin/xml2-config
configure:33305: will use system package and not install SPKG libxml2
...
configure:42387: checking for PRIMESIEVE
configure:42394: $PKG_CONFIG --exists --print-errors "primesieve >= 8.0"
./configure: /mnt/c/Strawberry/perl/bin/pkg-config: perl: bad
interpreter: No such file or directory
}}}

URL: https://trac.sagemath.org/34889
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Dima Pasechnik

README.md

@@ -228,16 +228,37 @@ in the Installation Guide.
     (If the bootstrapping prerequisites are not installed, this command will
     download a package providing pre-built bootstrap output instead.)
 
-6.  [macOS with homebrew] Set required environment variables for the build:
+6.  Sanitize the build environment. Use the command
 
-        $ source ./.homebrew-build-env
+        $ env
 
-    This is to make some of Homebrew's packages (so-called keg-only packages)
-    available for the build. Run it once to apply the suggestions for the current
-    terminal session. You may need to repeat this command before you rebuild Sage
-    from a new terminal session, or after installing additional homebrew packages.
-    (You can also add it to your shell profile so that it gets run automatically
-    in all future sessions.)
+    to inspect the current environment variables, in particular `PATH`,
+    `PKG_CONFIG_PATH`, `LD_LIBRARY_PATH`, `CFLAGS`, `CPPFLAGS`, `CXXFLAGS`,
+    and `LDFLAGS` (if set).
+
+    Remove items from these (colon-separated) environment variables
+    that Sage should not use for its own build. In particular, remove
+    items if they refer to a previous Sage installation.
+
+    - [WSL] In particular, WSL imports many items from the Windows
+      `PATH` variable into the Linux environment, which can lead to
+      confusing build errors. These items typically start with `/mnt/c`.
+      It is best to remove all of them from the environment variables.
+      For example, you can set `PATH` using the command:
+
+            $ export PATH=/usr/sbin/:/sbin/:/bin/:/usr/lib/wsl/lib/
+
+    - [macOS with homebrew] Set required environment variables for the build:
+
+            $ source ./.homebrew-build-env
+
+      This is to make some of Homebrew's packages (so-called keg-only
+      packages) available for the build. Run it once to apply the
+      suggestions for the current terminal session. You may need to
+      repeat this command before you rebuild Sage from a new terminal
+      session, or after installing additional homebrew packages.  (You
+      can also add it to your shell profile so that it gets run
+      automatically in all future sessions.)
 
 7.  Optionally, decide on the installation prefix (`SAGE_LOCAL`):