Minor make bug on AIX

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POSTGRESQL BUG REPORT TEMPLATE
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Your name : Pete Forman
Your email address : pete(dot)forman(at)westgeo(dot)com

System Configuration
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Architecture (example: Intel Pentium) : RS6000 POWER3

Operating System (example: Linux 2.0.26 ELF) : AIX 4.3.2

PostgreSQL version (example: PostgreSQL-7.0): PostgreSQL-7.0.3

Compiler used (example: gcc 2.8.0) : xlc 3.6.6

Please enter a FULL description of your problem:
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"gmake" with no arguments fails. postgres.imp is made before objects
that it depends on. "gmake all" works.

Please describe a way to repeat the problem. Please try to provide a
concise reproducible example, if at all possible:
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gmake

If you know how this problem might be fixed, list the solution below:
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Makefile.ports (copied from makefiles/Makefile.aix) is included before
the default target in GNUmakefile is reached. This patch calls that
default target.