Issue 1086: add support for generic batch build actions, and

specific support for batched compilation for Microsoft Visual C/C++.

Merged revisions 3819-3851,3854-3869,3871-3877,3880 via svnmerge from
http://scons.tigris.org/svn/scons/branches/sgk_batch

........
  r3820 | stevenknight | 2008-12-09 23:59:14 -0800 (Tue, 09 Dec 2008) | 6 lines

  Issue 1086:  Batch compilation support:
  * $MSVC_BATCH to control Visual C/C++ batch compilation.
  * New $CHANGED_SOURCES, $CHANGED_TARGETS, $UNCHANGED_SOURCES and
    $UNCHANGED_TARGETS construction variables.
  * New Action(batch_key=, targets=) keyword arguments.
........
  r3880 | stevenknight | 2009-01-07 20:50:41 -0800 (Wed, 07 Jan 2009) | 3 lines

  Use UniqueList objects to collect the all_children(), all_prerequisites()
  and all_sources() lists instead of calling uniquer_hashables() by hand.
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1 files changed, 20 insertions, 0 deletions

diff --git a/src/engine/SCons/Tool/msvc.xml b/src/engine/SCons/Tool/msvc.xml
index a2fdd7e..31dcdf1 100644
--- a/src/engine/SCons/Tool/msvc.xml
+++ b/src/engine/SCons/Tool/msvc.xml
@@ -140,6 +140,26 @@ env['CCPDBFLAGS'] = '/Zi /Fd${TARGET}.pdb'
 </summary>
 </cvar>
 
+<cvar name="MSVC_BATCH">
+<summary>
+When set to any true value,
+specifies that &SCons; should batch
+compilation of object files
+when calling the Microsoft Visual C/C++ compiler.
+All compilations of source files from the same source directory
+that generate target files in a same output directory
+and were configured in &SCons; using the same construction environment
+will be built in a single call to the compiler.
+Only source files that have changed since their
+object files were built will be passed to each compiler invocation
+(via the &cv-link-CHANGED_SOURCES; construction variable).
+Any compilations where the object (target) file base name
+(minus the <filename>.obj</filename>)
+does not match the source file base name
+will be compiled separately.
+</summary>
+</cvar>
+
 <cvar name="PCH">
 <summary>
 The Microsoft Visual C++ precompiled header that will be used when compiling