refine use of "Unix" in the installation instructions

Based on a suggestion from enedil <enedil@outlook.com>, avoid
suggesting that OS X is not Unix or that Linux is truly Unix.

INSTALL.txt

@@ -1,8 +1,8 @@
 Build Options
 =============
 
-For Unix platforms, instead of using this source repository, consider
+Instead of using this source repository, consider getting source for
-getting source for the current Racket release from
+the current Racket release from
 
   http://download.racket-lang.org/
 
@@ -16,7 +16,9 @@ documentation are pre-built.
 
 In contrast to this repository, release and snapshot source
 distributions will work in the
+
   configure --prefix=... && make && make install
+
 way that you probably expect.
 
 
@@ -31,7 +33,7 @@ If you stick with this repository, then you have several options:
    Instructions: In-place Build" below.
 
  * Unix-style install --- installs to a given destination directory
-   (Unix and Mac OS X, only), leaving no reference to the source
+   (on platforms other Windows), leaving no reference to the source
    directory. This is the most natural mode for installing once from
    the source repository. See "Quick Instructions: Unix-style Install"
    below.
@@ -51,8 +53,8 @@ If you stick with this repository, then you have several options:
 Quick Instructions: In-place Build
 ==================================
 
-On Unix, `make' (or `make in-place') creates a build in the "racket"
+On Unix (including Linux) and Mac OS X, `make' (or `make in-place')
-directory.
+creates a build in the "racket" directory.
 
 On Windows with Microsoft Visual Studio (any version between 2008/9.0
 and 2013/12.0), `nmake win32-in-place' creates a build in the "racket"
@@ -74,10 +76,10 @@ See "More Instructions: Building Racket" below for more information.
 Quick Instructions: Unix-style Install
 ======================================
 
-On Unix, `make unix-style PREFIX=<dir>' builds and installs into
+On Unix (including Linux), `make unix-style PREFIX=<dir>' builds and
-"<dir>" (which must be an absolute path) with binaries in "<dir>/bin",
+installs into "<dir>" (which must be an absolute path) with binaries
-packages in "<dir>/share/racket/pkgs", documentation in
+in "<dir>/bin", packages in "<dir>/share/racket/pkgs", documentation
-"<dir>/share/racket/doc", etc.
+in "<dir>/share/racket/doc", etc.
 
 On Mac OS X, `make unix-style PREFIX=<dir>' builds and installs into
 "<dir>" (whichmust be an absolute path) with binaries in "<dir>/bin",
@@ -305,8 +307,8 @@ where <dist-base> defaults to "racket" (but can be set via
 normalizing the Windows results to "i386-win32" and "x86_63-win32",
 -<dist-suffix> is omitted unless a `#:dist-suffix' string is specified
 for the client in the site configuration, and <ext> is
-platform-specific: ".sh" for Unix, ".dmg" or ".pkg" for Mac OS X, and
+platform-specific: ".sh" for Unix (including Linux), ".dmg" or ".pkg"
-".exe" for Windows.
+for Mac OS X, and ".exe" for Windows.
 
 Generating Installer Web Sites
 ------------------------------
@@ -475,8 +477,8 @@ In more detail:
      To change the base name of the installer file, provide `DIST_BASE'
      to `make'. The default is "racket".
 
-     To change the directory name for Unix installation, provide
+     To change the directory name for installation on Unix (including
-     `DIST_DIR' to `make'. The default is "racket".
+     Linux), provide `DIST_DIR' to `make'. The default is "racket".
 
      To add an extra piece to the installer's name, such as an
      identifier for a variant of Linux, provide `DIST_SUFFIX' to