Feature: add support for pkg-config for GEOS C API (only)

[mwtoews]

This feature adds support for pkg-config, which is a helper tool used when compiling applications and libraries on *nix systems. It is similar to geos-config, but is more widespread and generic.

Take a look what you might already have, which should already include proj and gdal:

pkg-config --list-all

Here is how it works from the command line:

export PKG_CONFIG_PATH=/path/to/geos/prefix
pkg-config geos --modversion  # same as `geos-config --version`
pkg-config geos --cflags  # same as `geos-config --cflags`
pkg-config geos --libs  # same as `geos-config --clibs`

Or from autotools files for a custom project "myapp":

# configure.ac
PKG_CHECK_MODULES([GEOS], [geos])

# Makefile.am 
myapp_CFLAGS = $(GEOS_CFLAGS)
myapp_LDADD = $(GEOS_LIBS)

---

Some considerations:

• It only links to the GEOS C API, by design
• It can be installed from the autotools or cmake setups, as it's a simple configuration file
• Static compilation has not been investigated yet, but could be. It's just a matter of adding .private sections to link in the configuration file to link any other libraries.
• For a future release, consider deprecating geos-config, which is a custom tool with a similar purpose.

xref: https://trac.osgeo.org/geos/ticket/1073