Fix some generator crashes on undefined CMAKE_BUILD_TYPE

Since commit 84fdc9921 (stringapi: Pass configuration names as strings,
2014-02-09), it is not safe to use GetDefinition("CMAKE_BUILD_TYPE")
without checking the return value.  Use GetSafeDefinition instead so
that a missing definition is treated as an empty string like code paths
did prior to the above commit.

Reported-by: Richard Wirth <richard@califax.de>
Fortran: Add support for GNU >= 4.9 compressed modules (#14975)

From the GCC 4.9 release notes for Fortran:

 https://gcc.gnu.org/gcc-4.9/changes.html
 "Module files: The version of the module files (.mod) has been
  incremented; additionally, module files are now compressed."

Teach cmDependsFortran::ModulesDiffer to look for the gzip magic numbers
at the beginning of the module file.  If found, assume the module was
produced by gfortran >= 4.9.  The modules do not appear to contain the
date as earlier versions did so we can compare the content directly
and do not actually need to decompress.
cmake: Add '-E env' command-line tool

Extend the cmake command-line interface to support

 cmake -E env [--unset=NAME]... [NAME=VALUE]... COMMAND [ARG]...

This will be useful to run processes with modified environments
without using a shell or a full "cmake -P" script to wrap it.

Extend the RunCMake.CommandLine test to cover success and failure cases.

Inspired-by: Jonathan Bohren <jbo@jhu.edu>
Merge topic 'cmake-E-env'

7abd5747 cmake: Add '-E env' command-line tool
Merge topic 'stringapi-crash-cleanup'

c491cb1e Fix some generator crashes on undefined CMAKE_BUILD_TYPE
Merge topic 'gfortran-compressed-modules'

d90be200 Fortran: Add support for GNU >= 4.9 compressed modules (#14975)