Fix build failures on Fedora

— FCFLAGS needs to contain the compilation flags used when compiling Octave,
  otherwise it fails at configure stage when looking for gfortran
— Explicitly add -L$OCTLIBDIR, because with Octave 5 it is no longer there, and
  on Fedora the Octave libraries are no in the default linker search path

mex/build/octave/configure.ac

@@ -28,7 +28,7 @@ test "$MKOCTFILE" = no && AC_MSG_ERROR([Octave cannot be found])
 CC=$($MKOCTFILE -p CC)
 CXX=$($MKOCTFILE -p CXX)
 CFLAGS="$($MKOCTFILE -p CFLAGS) -Wall -Wno-parentheses"
-FCFLAGS="-g -O2 -Wall"
+FCFLAGS="$($MKOCTFILE -p FFLAGS) -Wall -std=gnu" # Override -std=legacy that is in FFLAGS in Octave 5
 FFLAGS="$($MKOCTFILE -p FFLAGS) -Wall"
 CXXFLAGS="$($MKOCTFILE -p CXXFLAGS) -Wall -Wno-parentheses -Wold-style-cast"
 LDFLAGS="$($MKOCTFILE -p LFLAGS) $($MKOCTFILE -p LDFLAGS)"

mex/build/octave/mex.am

@@ -7,9 +7,9 @@ DEFS += -DMEXEXT=\".mex\"
 
 AM_CFLAGS = $(shell $(MKOCTFILE) -p CPICFLAG)
 AM_FFLAGS = $(shell $(MKOCTFILE) -p FPICFLAG)
-AM_FCFLAGS = -fPIC
+AM_FCFLAGS = $(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)