pgsql: Add FAQ text about Makefile.custom:

Log Message:
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Add FAQ text about Makefile.custom:

<P><I>src/Makefile.custom</I> can be used to set environment variables,
like <I>CUSTOM_COPT</I>, that are used for every compile.

Modified Files:
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pgsql/doc:
FAQ_DEV (r1.121 -> r1.122)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/FAQ_DEV.diff?r1=1.121&amp;r2=1.122)
pgsql/doc/src/FAQ:
FAQ_DEV.html (r1.125 -> r1.126)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/FAQ_DEV.html.diff?r1=1.125&amp;r2=1.126)