pgsql: Make pkg-config files cross-compile friendly

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(at)gmx(dot)net>
Co-authored-by: Peter Eisentraut <peter(dot)eisentraut(at)enterprisedb(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/20200305213827.GA25135%40darth.lan

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/6588d8416e4ef84fd99fb271b63116f207c6c479

Modified Files
--------------
src/Makefile.shlib | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)