Add a note about the state of C++ support

INSTALL.swift

@@ -34,13 +34,17 @@ directory. See README for run parameters.
 
 SWIFT has been successfully built and tested with the following compilers:
 
-  - GCC 4.8.x  
+  - GCC 4.8.x
   - Intel ICC 15.0.x
-  - clang 3.4.x 
+  - clang 3.4.x
 
 More recent versions and slightly older ones should also be able to
 build the software.
 
+It has also been built with Intel and GNU C++ compilers, but that currently
+requires the --disable-vec and, for Intel, --disable-compiler-warnings
+configure options.
+
 By default an attempt to choose suitable set of optimizing compiler flags
 will be made, targeted for the host machine of the build. If this doesn't
 work or the binaries will for another architecture then you can stop the
@@ -61,7 +65,7 @@ You could also add some additional flags:
 
     ./configure --enable-debug --disable-optimization CFLAGS="-O2"
 
-for instance. GCC address sanitizer flags can be included using the 
+for instance. GCC address sanitizer flags can be included using the
 
     ./configure --enable-sanitizer
 
@@ -113,7 +117,7 @@ before you can build it.
 	none-standard name.
 
 
- - libtool: The build system relies on libtool.
+ - libtool: The build system relies on libtool as well as the other autotools.
 
 
                            Optional Dependencies