Fails to build with GCC 7

[sjackman]

    porechop/include/seqan/system/file_sync.h: In member function 'bool seqan::File<seqan::Sync<TSpec> >::openTemp(int)':
    porechop/include/seqan/system/file_sync.h:337:25: error: call of overloaded 'empty(std::__cxx11::string&)' is ambiguous
             if (empty(tmpDir))
                             ^

See bioconda/bioconda-recipes#11399
and https://circleci.com/gh/bioconda/bioconda-recipes/30016

[e73kiel]

OS=Centos 7.6
Python version=Python 3.6.3
gcc version=gcc 7.3.0

I updated

Porechop/porechop/include/seqan
with
seqan/include/seqan/

and installation finished without error.

[dpryan79]

This can partially be solved by using a more recent seqan, but that results in a couple failed unit tests:

https://circleci.com/gh/bioconda/bioconda-recipes/69713

See bioconda/bioconda-recipes#16212 for an example. I'm not sure about the changes in the seqan versions and how those relate to this.