How to install pgxn with correct PostgreSQL version?

On sudo -H pgxn install semver, ERROR about wrong PostgreSQL version (it is v10 not v12),

INFO: best version: semver 0.20.3
INFO: saving /tmp/tmpba6hta5a/semver-0.20.3.zip
INFO: unpacking: /tmp/tmpba6hta5a/semver-0.20.3.zip
INFO: building extension
gcc -Wall -Wmissing-prototypes -Wpointer-arith... src/semver.c: In function ‘tail_cmp’:
src/semver.c:440:3: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
   char *dot = ".";
   ^
gcc -Wall -Wmissing-prototypes -...
/usr/bin/install -c -m 644 .//doc/semver.mmd '/usr/share/doc/postgresql-doc-12/extension/'

INFO: installing extension
/bin/mkdir -p '/usr/share/postgresql/12/extension'
/bin/mkdir -p '/usr/share/postgresql/12/extension'
/bin/mkdir -p '/usr/lib/postgresql/12/lib'
/bin/mkdir -p '/usr/share/doc/postgresql-doc-12/extension'
/usr/bin/install -c -m 644 .//semver.control '/usr/share/postgresql/12/extension/'
/usr/bin/install -c -m 644 .//sql/semver--0.20.0.sql ...

I try also to fix "postgresql headers", by this clue using sudo apt install postgresql-server-dev-10, but no effect.

I was using the pgxn.org extensions, that has no obvious "installation Guide" on its home. Seems that the Guide is pgxn.github.io/pgxnclient, but it say nothing about change PostgreSQL configs (but Pg Guide about extend-pgxs perhaps say something).

More details on json_fdw/issues and this other question.

ENV details

• on psql "postgres://postgres:passwd@localhost/myBase" -c "select version()" = PostgreSQL 10.12

• old server UBUNTU 16 LTS

• psql --version = 12.2 (but SQL is 10.12!)

• pgxn installed by sudo easy_install pgxnclient.
  redo by sudo -H pip install --upgrade pgxnclient
pgxn --version = pgxnclient 1.3

• for json_fdw package prepared, before pgxn install, with sucess sudo apt install zlib1g-dev libyajl-dev.

Seems the real problem is a PostgreSQL version confusion: the two exists, ls /usr/share/postgresql/10/extension and ls /usr/share/postgresql/12/extension