Cannot build or-tools with USE_SCIP=OFF

[pfetsch]

What version of OR-tools and what language are you using?
Version: stable
Language: C++

What operating system (Linux, Windows, ...) and version?
Linux, Ubuntu 18.04

What did you do?
Steps to reproduce the behavior:

1. Clone or-tools: git clone https://github.com/google/or-tools.git`
2. make USE_SCIP=OFF third_party
3. This produces the error:

PKG_CONFIG_PATH=tmp/build/or-tools/dependencies/install/lib64/pkgconfig:tmp/build/or-tools/dependencies/install/lib/pkgconfig:tmp/build/or-tools/dependencies/install/share/pkgconfig::tmp/build/or-tools/dependencies/install/lib/pkgconfig \
pkg-config --libs cbc > tmp/build/or-tools/dependencies/install/share/coin/doc/Cbc/cbc_addlibs.txt
make[5]: Leaving directory 'tmp/build/or-tools/dependencies/sources/Cbc-2.10.5/Cbc'
make[4]: Leaving directory 'tmp/build/or-tools/dependencies/sources/Cbc-2.10.5/Cbc'
make[3]: Leaving directory 'tmp/build/or-tools/dependencies/sources/Cbc-2.10.5/Cbc'
make[2]: Leaving directory 'tmp/build/or-tools/dependencies/sources/Cbc-2.10.5/Cbc'
make[2]: Entering directory tmp/build/or-tools/dependencies/sources/Cbc-2.10.5'
make[3]: Entering directory tmp/build/or-tools/dependencies/sources/Cbc-2.10.5'
make[3]: Nothing to be done for 'install-exec-am'.
make[3]: Nothing to be done for 'install-data-am'.
make[3]: Leaving directory 'tmp/build/or-tools/dependencies/sources/Cbc-2.10.5'
make[2]: Leaving directory 'tmp/build/or-tools/dependencies/sources/Cbc-2.10.5'
make[1]: Leaving directory 'tmp/build/or-tools/dependencies/sources/Cbc-2.10.5'
touch ortools/gen/ortools/linear_solver/lpi_glop.cc
touch: cannot touch 'ortools/gen/ortools/linear_solver/lpi_glop.cc': No such file or directory
makefiles/Makefile.third_party.unix.mk:618: recipe for target 'ortools/gen/ortools/linear_solver/lpi_glop.cc' failed
make: *** [ortools/gen/ortools/linear_solver/lpi_glop.cc] Error 1

4. Indeed, the path ortools/gen does not exist at this stage. Moreover, it should not be build, since this file depends on SCIP.
5. Then I tried make USE_SCIP=OFF USE_COINOR=OFF third_party. This works.
6. However, then make USE_SCIP=OFF USE_COINOR=OFF all results in:

mkdir ortools/gen/ortools/gscip
LD_LIBRARY_PATH="tmp/build/or-tools/dependencies/install/lib64":"tmp/build/or-tools/dependencies/install/lib": tmp/build/or-tools/dependencies/install/bin/protoc --proto_path=. -Itmp/build/or-tools/dependencies/install/include --cpp_out=ortools/gen ./ortools/gscip/gscip.proto
touch ortools/gen/ortools/gscip/gscip.pb.h
mkdir -p objs/gscip
g++ -fPIC -std=c++17 -fwrapv -O4 -DNDEBUG -I. -Iortools/gen -Wno-deprecated -DUSE_GLOP -DUSE_BOP    -Itmp/build/or-tools/dependencies/install/include -Itmp/build/or-tools/dependencies/install/include -DOR_TOOLS_MAJOR=8 -DOR_TOOLS_MINOR=1 -c ./ortools/gscip/gscip.cc -o objs/gscip/gscip.o
In file included from ./ortools/gscip/gscip.cc:14:0:
./ortools/gscip/gscip.h:61:10: fatal error: scip/scip.h: No such file or directory
 #include "scip/scip.h"
          ^~~~~~~~~~~~~
compilation terminated.

7. Again this file depends on SCIP and should be build.