It turns out that CMake always canonicalizes `CMAKE_INSTALL_PREFIX` to
an absolute path--if it's a relative path, it canonicalizes it relative
to the build directory. As a result, the only thing the DESTDIR and
relative directory check prevents is an installation into the root
directory since CMake strips the trailing slash, turning "/" into an
empty string. Let's just remove the check all together, since it cannot
accomplish what we intended.
It turns out that `file(INSTALL ...)` already accounts for `DESTDIR`, so
this wasn't creating the directory structure in the correct location.
Instead, we need to do our existence check with `DESTDIR`, but leave it
off when doing the install step.
While we're at it, add a check to make sure `ENV{DESTDIR}` is not being
used with a relative path, as that construct doesn't make much sense.
This fixes issue #1387 discovered while trying to make helptag
generation work correctly in #1381.
The install() command will create the parent directories, but it does so
with the user's umask. We want to do our best to make sure the correct
permissions are being set, without clobbering existing permissions.
To do this, this commit introduces an install_helper(), which is similar
in signature to the install() command, to help ensure that directories
are created ahead of the actual install() command. This will attempt to
use 0644 permissions for files and 0755 permissions for directories by
default--though they can be overridden.
To make this work correctly, without trying to introduce some mechanism
with setting the umask, it meant that there's a small portion that makes
use of an "internal" version of the file() command. It has been tested
on CMake 2.8.11, 2.8.12, and 3.0.2, and works correctly on all versions.
This fixes#1201 and #1086.
