During installation of octave-devel the package umfpack-4.4 is installed. Later in the building process ufsparse-2.1.1_1 stops the building with:

downdate: time:     0.01 (incl solve) resid: 3.64e-16
gmake[2]: Leaving directory `/usr/ports/math/ufsparse/work/SuiteSparse/CXSparse/Demo'
gmake[1]: Leaving directory `/usr/ports/math/ufsparse/work/SuiteSparse/CXSparse'
===>  Installing for ufsparse-2.1.1_1

===>  ufsparse-2.1.1_1 conflicts with installed package(s):
      umfpack-4.4

      They install files into the same place.
      Please remove them first with pkg_delete(1).
*** Error code 1

Stop in /usr/ports/math/ufsparse.
*** Error code 1

Stop in /usr/ports/math/octave-devel.
** Command failed [exit code 1]: /usr/bin/script -qa /tmp/portinstall.4190.0 env make
** Fix the problem and try again.
** Listing the failed packages (*:skipped / !:failed)
        ! math/octave-devel     (unknown build error)
--->  Packages processed: 0 done, 0 ignored, 0 skipped and 1 failed

As you can see during install umfpack is in the dependecy list:

7of9# portinstall math/octave-devel
[Updating the pkgdb <format:bdb_btree> in /var/db/pkg ... - 511 packages found (-0 +3) ... done]
--->  Installing 'octave-devel-2.9.8' from a port (math/octave-devel)
--->  Building '/usr/ports/math/octave-devel'
===>  Cleaning for perl-5.8.8
===>  Cleaning for gnuplot-4.0.0_4
===>  Cleaning for gperf-3.0.1
===>  Cleaning for umfpack-4.4       <------ HERE
===>  Cleaning for glpk-4.10
===>  Cleaning for ufsparse-2.1.1_1  <------ and HERE
===>  Cleaning for gmake-3.81_1
===>  Cleaning for autoconf-2.59_2
===>  Cleaning for fftw3-3.1.2
===>  Cleaning for atlas-3.6.0,1
===>  Cleaning for hdf5-1.6.5
===>  Cleaning for gd-2.0.33_4,1
===>  Cleaning for pdflib-6.0.3
===>  Cleaning for png-1.2.12_1
===>  Cleaning for xorg-libraries-6.9.0
===>  Cleaning for blas-1.0
===>  Cleaning for metis-4.0
===>  Cleaning for gettext-0.14.5_2
===>  Cleaning for m4-1.4.4
===>  Cleaning for help2man-1.36.4_1
===>  Cleaning for pkg-config-0.21
===>  Cleaning for jpeg-6b_4
===>  Cleaning for freetype2-2.1.10_5
===>  Cleaning for libtool-1.5.22_2
===>  Cleaning for imake-6.9.0
===>  Cleaning for libdrm-2.0.2
===>  Cleaning for fontconfig-2.3.2_5,1
===>  Cleaning for libiconv-1.9.2_2
===>  Cleaning for p5-gettext-1.05_1
===>  Cleaning for expat-2.0.0_1
===>  Cleaning for octave-devel-2.9.8
===>  Vulnerability check disabled, database not found
===>  Extracting for octave-devel-2.9.8
=> MD5 Checksum OK for octave-2.9.8.tar.bz2.
=> SHA256 Checksum OK for octave-2.9.8.tar.bz2.

Fix: 

Work-around: manually deinstall umfpack-4.4

# pkg_deinstall umfpack-4.4

and the install octave with

# cd /usr/ports/math/octave-devel
# make install

Stop at the same stage. but after 

# pkg_deinstall umfpack-4.4

again, the installation of octave-devel continues with

# make install

Strange.
How-To-Repeat: Nothing special, just installing octave-devel using 
# portinstall math/octave-devel