scons: Build progs together with everything else.

This is a substantial reorganization, This particular commit enables:
- building the progs for unices platforms
- glew is now built as a shared library (it is the default, and it is
inconvenient and pointless to shift away from that default)
- all progs get built by default

docs/install.html

@@ -351,20 +351,11 @@ example linux or windows, <i>machine</i> is x86 or x86_64, optionally followed
 by -debug for debug builds.
 </p>
 
-<p>
-The sample programs are built seperately. To build them do
-<pre>
-    scons -C progs
-</pre>
-And the build output will be placed in progs/build/...
-</p>
-
 <p>
 To build Mesa with SCons for Windows on Linux using the MinGW crosscompiler toolchain do
 </p>
 <pre>
     scons platform=windows toolchain=crossmingw machine=x86 statetrackers=mesa drivers=softpipe,trace winsys=gdi
-    scons -C progs platform=windows toolchain=crossmingw machine=x86 -k
 </pre>
 <p>
 This will create: