Revision 1881958

Restoring support for building with APR 1.4.

Change the minimum required version of APR from 1.5 to 1.4.

This is required to build and package Subversion on CentOS 7 using the
distribution's APR package.  (The alternative of also packaging a later
version of APR and APR-util has undesirable consequences: libserf and apache
httpd would need to link against these too.)

The first part of this patch is a partial revert of r1874094 (which changed
the requirement from APR 1.3 to APR 1.5), reverting only the parts needed to
restore APR 1.4 compatibility.

* INSTALL,
  build/generator/gen_win_dependencies.py,
  configure.ac:
  Change minimum APR version from 1.5 to 1.4.

* subversion/include/svn_types.h,
  subversion/libsvn_subr/iter.c:
  Restore the compatibility versions of apr_hash_this_key,
  apr_hash_this_key_len, apr_hash_this_val.

The second part of this patch adds a local copy of the 'apr_escape_shell'
function, copied from APR 1.7.0, if building against APR < 1.5.

* subversion/include/private/svn_dep_compat.h
  (APR_ESCAPE_STRING, apr_escape_shell): Declare if APR < 1.5.

* subversion/libsvn_subr/apr_escape.c
  New, copied from parts of APR 1.7.0.

* subversion/libsvn_subr/cmdline.c
  When APR < 1.5, include 'svn_dep_compat.h' instead of 'apr_escape.h'.