cannot find PGXS file when installing an extension?

I'm using 11.0 on FreeBSD:

testdb=> select version();
version
---------------------------------------------------------------------------------------------------------
PostgreSQL 11.0 on x86_64-unknown-freebsd11.1, compiled by gcc
(FreeBSD Ports Collection) 6.4.0, 64-bit

I've a very basic extension made by my own:

cleopatra% ls
Makefile fluca--1.0.sql fluca.control

cleopatra% cat Makefile
EXTENSION = fluca
DATA = fluca--1.0.sql

PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)

but when I launch make I got an error about a "cannot find" on the
line of the include directive:

cleopatra% sudo make install
make: "/usr/home/luca/tmp/fluca/Makefile" line 6: Could not find
make: Fatal errors encountered -- cannot continue
make: stopped in /usr/home/luca/tmp/fluca

but seems to me everything is in place:

cleopatra% pg_config --pgxs
/opt/pg11/lib/postgresql/pgxs/src/makefiles/pgxs.mk

I've then tried to force the inclusion of the file adding it to the Makefile:

cleopatra% cat Makefile
EXTENSION = fluca
DATA = fluca--1.0.sql

PG_CONFIG = pg_config
include /opt/pg11/lib/postgresql/pgxs/src/makefiles/pgxs.mk

which produces a verbose error output:

cleopatra% sudo make install
make: "/opt/pg11/lib/postgresql/pgxs/src/makefiles/pgxs.mk" line 63:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/makefiles/pgxs.mk" line 64:
Need an operator
make: Unknown modifier ' '
make: "/opt/pg11/lib/postgresql/pgxs/src/makefiles/pgxs.mk" line 65:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/makefiles/pgxs.mk" line 66:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/makefiles/pgxs.mk" line 67:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/makefiles/pgxs.mk" line 70:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 44:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 49:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 51:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 52:
Missing dependency operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 55:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 58:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 59:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 60:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 65:
Missing dependency operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 68:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 72:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 73:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 93:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 105:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 106:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 108:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 109:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 112:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 113:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 115:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 116:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 121:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 122:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 124:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 125:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 130:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 131:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 133:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 134:
Need an operator
make: "/opt/pg11/lib/postgresql/pgxs/src/Makefile.global" line 139:
Need an operator

What am I missing here?

Thanks,
Luca