Make pkg-config files cross-compile friendly

Currently the pc files use hard coded paths for "includedir" and
"libdir."

Example:

  Cflags: -I/usr/include
  Libs: -L/usr/lib -lpq

This is not very fortunate when cross compiling inside a buildroot,
where the includes and libs are inside a staging directory, because
this introduces host paths into the build:

  checking for pkg-config... /builder/shared-workdir/build/sdk/staging_dir/host/bin/pkg-config
  checking for PostgreSQL libraries via pkg_config... -L/usr/lib <----

This commit addresses this by doing the following two things:

  1. Instead of hard coding the paths in "Cflags" and "Libs"
     "${includedir}" and "${libdir}" are used.  Note: these variables
     can be overriden on the pkg-config command line
     ("--define-variable=libdir=/some/path").

  2. Add the variables "prefix" and "exec_prefix".  If "includedir"
     and/or "libdir" are using these then construct them accordingly.
     This is done because buildroots (for instance OpenWrt) tend to
     rename the real pkg-config and call it indirectly from a script
     that sets "prefix", "exec_prefix" and "bindir", like so:

     pkg-config.real --define-variable=prefix=${STAGING_PREFIX} \
       --define-variable=exec_prefix=${STAGING_PREFIX} \
       --define-variable=bindir=${STAGING_PREFIX}/bin $@

Example #1: user calls ./configure with "--libdir=/some/lib" and
"--includedir=/some/include":

  prefix=/usr/local/pgsql
  exec_prefix=${prefix}
  libdir=/some/lib
  includedir=/some/include

  Name: libpq
  Description: PostgreSQL libpq library
  Url: http://www.postgresql.org/
  Version: 12.1
  Requires:
  Requires.private:
  Cflags: -I${includedir}
  Libs: -L${libdir} -lpq
  Libs.private:  -lcrypt -lm

Example #2: user calls ./configure with no arguments:

  prefix=/usr/local/pgsql
  exec_prefix=${prefix}
  libdir=${exec_prefix}/lib
  includedir=${prefix}/include

  Name: libpq
  Description: PostgreSQL libpq library
  Url: http://www.postgresql.org/
  Version: 12.1
  Requires:
  Requires.private:
  Cflags: -I${includedir}
  Libs: -L${libdir} -lpq
  Libs.private:  -lcrypt -lm

Like this the paths can be forced into the staging directory when
using a buildroot setup:

  checking for pkg-config... /home/sk/tmp/openwrt/staging_dir/host/bin/pkg-config
  checking for PostgreSQL libraries via pkg_config... -L/home/sk/tmp/openwrt/staging_dir/target-mips_24kc_musl/usr/lib

Author: Sebastian Kemper <sebastian_ml@gmx.net>
Co-authored-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/20200305213827.GA25135%40darth.lan

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