fix build on older gcc

[junghans]

Report on mailing list: https://groups.google.com/g/votca/c/l-5RSrxQegk/m/X1OcT_0gAwAJ

[junghans]

@votca-bot changelog: fix build on Ubuntu 16.04

[codecov]

Codecov Report

Merging #577 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #577   +/-   ##
======================================
  Coverage    57.3%   57.3%           
======================================
  Files         313     313           
  Lines       32388   32388           
======================================
  Hits        18566   18566           
  Misses      13822   13822           

Impacted Files	Coverage Δ
include/votca/xtp/molden.h	100.0% <ø> (ø)
include/votca/xtp/orbreorder.h	100.0% <ø> (ø)
src/libxtp/qmpackages/orca.h	100.0% <ø> (ø)
src/libxtp/qmpackages/xtpdft.h	0.0% <ø> (ø)

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[JensWehner]

Okay, I am not sure if we still support gcc 5.4 but I do not mind.

Thanks for the fix

[sandipan84]

I verify that I have made the above changes (mentioned by Christoph) to the files-- molden.h, orbreorder.h, orca.h and xtpdft.h for installing VOTCA in my Ubuntu 16.04 desktop.