Fix build failures on Fedora

Explicitly add -L$OCTLIBDIR, because with Octave 5 it is no longer there, and
on Fedora the Octave libraries are not in the default linker search path

(manually cherry picked from commit 3a42c234)

mex/build/octave/mex.am

@@ -8,7 +8,7 @@ DEFS += -DMEXEXT=\".mex\"
 AM_CFLAGS = $(shell $(MKOCTFILE) -p CPICFLAG)
 AM_FFLAGS = $(shell $(MKOCTFILE) -p FPICFLAG)
 AM_CXXFLAGS = $(shell $(MKOCTFILE) -p CXXPICFLAG)
-AM_LDFLAGS = $(shell $(MKOCTFILE) -p DL_LDFLAGS)
+AM_LDFLAGS = $(shell $(MKOCTFILE) -p DL_LDFLAGS) -L"$(shell $(MKOCTFILE) -p OCTLIBDIR)"
 
 LIBS += $(shell $(MKOCTFILE) -p OCTAVE_LIBS)
 LIBS += $(shell $(MKOCTFILE) -p BLAS_LIBS)