Travis cannot find serd lib

[RubenVerborgh]

From https://github.com/rdfhdt/hdt-cpp/pull/94#issuecomment-332876008:

removing the --with-raptor2-prefix flag breaks the inclusion of Serd on Travis:

FAIL: bit375


/home/travis/build/LaurensRietveld/hdt-cpp/libhdt/tests/.libs/lt-bit375: error while loading shared libraries: libserd-0.so.0: cannot open shared object file: No such file or directory

FAIL: logarr


/home/travis/build/LaurensRietveld/hdt-cpp/libhdt/tests/.libs/lt-logarr: error while loading shared libraries: libserd-0.so.0: cannot open shared object file: No such file or directory

FAIL: streamtest


/home/travis/build/LaurensRietveld/hdt-cpp/libhdt/tests/.libs/lt-streamtest: error while loading shared libraries: libserd-0.so.0: cannot open shared object file: No such file or directory

[ptorrestr]

For some reason -lserd-0 is not getting found on Linux. I made a small change in travis.yml to force the paths (#98)

I will explore a more elegant solution.

[drobilla]

Should generally do what pkg-configuration says, but it's been a while since I dealt with local autoconf things. I'll look into it, but ramming a path in the environment shouldn't hurt anything

[LaurensRietveld]

I've merged the PR (#98) with the workaround (forcing the paths). Let's undo this workardoung when we've fixed the serd reference