A few sweeping changes were needed:

  - Avoid use of HEREDOC, which does not seem to work.
  - Avoid extra '.' in paths by using '_cmk' and '_tmp'
    instead of '.cmk' and '.tmp'.

This adds the Modules/Platform/OpenVMS.cmake platform file for OpenVMS.
We just use Unix-like rules to work with the GNV compiler front-end.

A problem with process execution currently prevents CMake link scripts
from working, so we avoid using them.

This shell does not exist on VMS, so we leave it out.

The VMS posix path emulation does not handle multiple '.' characters in
file names in all cases.  This avoids adding extra '.'s to file and
directory names for target directories and generated files.

This teaches ConvertToUnixSlashes to convert VMS paths into posix-style
paths.  We also set the DECC$FILENAME_UNIX_ONLY feature so the process
always sees posix-style paths on disk.

The Compaq compiler's std::unique algorithm followed by deletion of the
extra elements seems to crash.  For now we'll accept the duplicate
dependencies on this platform.

The mknod and mkfifo functions are not available on VMS.

This defines IOCTL_3_ARGS in 'cmcurl/setup.h' to teach curl sources
about the three-argument ioctl() on VMS.

The value is computed by a try-compile for libtar.  This teaches the
sources to actually use the result.

This achieves basic process execution on OpenVMS.  We use work-arounds
for different fork()/exec() behavior and a lack of select().

VMS emulates fork/exec using setjmp/longjmp to evaluate the child and
parent return cases from fork.  Therefore both must be invoked from the
same function.

Since select() works only for sockets we use the BeOS-style polling
implementation.  However, non-blocking reads on empty pipes cannot be
distinguished easily from the last read on a closed pipe.  Therefore we
identify end of data by an empty read after the child terminates.

This avoids use of an uninitialized value in the KWSys ProcessUNIX
polling implementation when no timeout is given.

The Compaq compiler (on VMS) includes 'String.c' in source files that
use the stl string while looking for template definitions.  This was the
true cause of double-inclusion of the 'kwsysPrivate.h' header.  We work
around the problem by conditionally compiling the entire source file on
a condition only true when really building the source.

HP-UX uses both .sl and .so as extensions for shared libraries.  This
teaches CMake to recognize .so shared libraries so they are treated
properly during link dependency analysis.

Alex

This enables the --enable-auto-import linker flag on Cygwin when linking
executables.  It works with the old gcc 3.x compiler and is necessary
for the new gcc 4.x compiler.  See issue #9071.

BUG: Rearrange paths to find correct installations of Visual Studio. Patch devenv.modified_search_order.patch came from issue #7919.