Reference java source files with absolute path in Makefile.

[mbanck-cd]

This allows for out-of-tree builds, fixing issue #39.

[MinhLA1410]

Hello @mbanck-ntap ,
I checked your PR and it tried to use new Makefile variable srcdir when build jdbc_fdw with Postgres source tree
The error happens.
Maybe srcdir is a global variable of postgres, could you please check it?

Proceduce:

cd home/vagrant/workplace/postgresql-xxx/contrib
git clone path/to/jdbc_fdw
cd jdbc_fdw
make srcdir=/home/vagrant/workplace/postgresql-xxx/contrib/jdbc_fdw

Output:

[vagrant@localhost jdbc_fdw]$ make srcdir=/home/vagrant/workplace/postgresql-xxx/contrib/jdbc_fdw
make -C ../../src/backend generated-headers
make[1]: Entering directory '/home/vagrant/workplace/postgresql-xxx/src/backend'
Makefile:26: /home/vagrant/workplace/postgresql-xxx/contrib/jdbc_fdw/common.mk: No such file or directory
make[1]: *** No rule to make target '/home/vagrant/workplace/postgresql-xxx/contrib/jdbc_fdw/common.mk'.  Stop.
make[1]: Leaving directory '/home/vagrant/workplace/postgresql-xxx/src/backend'
make: *** [../../src/Makefile.global:384: submake-generated-headers] Error 2