added pg_config support; factored CFLAGS and LIBS as separate steps.

[mmuurr]

The PostgreSQL installations on Amazon Linux have a variety of annoying issues.

1. They don't play nicely with pkg-config, specifically libpq doesn't have any useful .pc entries.
2. The installation location is non-standard.

But (fortunately!), pg_config does work just fine... so, the proposed changes in this pull request are:

1. Factor the PKG_CFLAGS and PKG_LIBS into two separate steps (i.e. set the values for each one separately, and check for the cascading conditions below separately for each).
2. Cascade priority as follows (from highest in the list to lowest):

• INCLUDEDIR or LIBDIR found on the command line; then
• values retrieved from pkg-config, if available; then
• values retrieved from pg_config.

This solves the problem of screwed-up pkg-config entries more generally for Linux distributions outside just Amazon Linux.
And for Amazon Linux in particular, this solves the annoying installation issues.
(I've also included a separate message for Amazon Linux users since there is no generic postgresql-devel package... instead you must specify the version, e.g. postgresql94-devel.)

I'm open to suggestions for other ways to make pkg-config and pg_config play nicely together in the case where both might be installed on a single system, but the pkg-config entries be erroneous or missing (as is the case for most Amazon Linux users).

Closes #22.

[krlmlr]

Thanks, this looks good so far. I'd like to wait for feedback from other affected users before merging this.

[cilindrox]

Getting the following error when testing this

Loading required package: methods
Error in .valueClassTest(ans, "character", "dbListTables") :
  invalid value from generic function 'dbListTables', class "NULL", expected "
character"
Calls: db_list_tables ... db_list_tables.DBIConnection -> dbListTables -> .valueClassTest
In addition: Warning message:
In value[[3L]](cond) : could not find function "columnToRownames"
Execution halted

Which made me think of #88

Update: got this fixed by rebasing this PR on top of master

[krlmlr]

Thanks!