Allow PostgreSQL 12-to-be (master)

[df7cb]

I'd appreciate this check to be removed entirely. If it doesn't build+test with the new version, people notice anyway, but if it does work with the new version, this check just makes it fail for no reason.

That said, there's real problems ahead, building against 12beta1:

make[2]: Verzeichnis „/home/cbe/projects/postgresql/mysql-fdw/postgresql-mysql-fdw/build-12“ wird betreten
gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -Wno-format-truncation -Wno-stringop-truncation -g -g -O2 -fstack-protector-strong -Wformat -Werror=format-security -fno-omit-frame-pointer -g -O2 -fdebug-prefix-map=/home/cbe/projects/postgresql/mysql-fdw/postgresql-mysql-fdw=. -fstack-protector-strong -Wformat -Werror=format-security -fPIC -I/usr/include/mariadb -I/usr/include/mariadb/mysql -D _MYSQL_LIBNAME=\"libmysqlclient.so\" -I. -I/home/cbe/projects/postgresql/mysql-fdw/postgresql-mysql-fdw -I/usr/include/postgresql/12/server -I/usr/include/postgresql/internal  -Wdate-time -D_FORTIFY_SOURCE=2 -D_GNU_SOURCE -I/usr/include/libxml2  -I/usr/include/mit-krb5  -c -o connection.o /home/cbe/projects/postgresql/mysql-fdw/postgresql-mysql-fdw/connection.c
In file included from /home/cbe/projects/postgresql/mysql-fdw/postgresql-mysql-fdw/connection.c:17:
/home/cbe/projects/postgresql/mysql-fdw/postgresql-mysql-fdw/mysql_fdw.h:31:10: fatal error: nodes/relation.h: Datei oder Verzeichnis nicht gefunden
 #include "nodes/relation.h"
          ^~~~~~~~~~~~~~~~~~
compilation terminated.