DESTDIR environment variable is ignored during installation

[robert-scheck]

The DESTDIR environment variable is ignored during installation:

[…]
-- Installing: /builddir/build/BUILDROOT/dislocker-0.5.1-1.fc24.x86_64/usr/lib64/libdislocker.so.0.5.1
-- Installing: /builddir/build/BUILDROOT/dislocker-0.5.1-1.fc24.x86_64/usr/lib64/libdislocker.so.0.5
-- Installing: /builddir/build/BUILDROOT/dislocker-0.5.1-1.fc24.x86_64/usr/lib64/libdislocker.so
-- Installing: /builddir/build/BUILDROOT/dislocker-0.5.1-1.fc24.x86_64/usr/bin/dislocker-fuse
-- Removed runtime path from "/builddir/build/BUILDROOT/dislocker-0.5.1-1.fc24.x86_64/usr/bin/dislocker-fuse"
-- Installing: /builddir/build/BUILDROOT/dislocker-0.5.1-1.fc24.x86_64/usr/share/man/man1/dislocker-fuse.1.gz
-- Installing: /builddir/build/BUILDROOT/dislocker-0.5.1-1.fc24.x86_64/usr/bin/dislocker-file
-- Removed runtime path from "/builddir/build/BUILDROOT/dislocker-0.5.1-1.fc24.x86_64/usr/bin/dislocker-file"
-- Installing: /builddir/build/BUILDROOT/dislocker-0.5.1-1.fc24.x86_64/usr/share/man/man1/dislocker-file.1.gz
-- Installing: /builddir/build/BUILDROOT/dislocker-0.5.1-1.fc24.x86_64/usr/bin/dislocker-metadata
-- Removed runtime path from "/builddir/build/BUILDROOT/dislocker-0.5.1-1.fc24.x86_64/usr/bin/dislocker-metadata"
-- Installing: /builddir/build/BUILDROOT/dislocker-0.5.1-1.fc24.x86_64/usr/bin/dislocker-bek
-- Removed runtime path from "/builddir/build/BUILDROOT/dislocker-0.5.1-1.fc24.x86_64/usr/bin/dislocker-bek"
failed to create symbolic link '/usr/bin/dislocker': Permission denied
failed to create symbolic link '/usr/share/man/man1/dislocker.1.gz': Permission denied
[…]

[Aorimn]

Thanks again for the PR.