configure: Support for libpng-config

[HuidaeCho]

This PR is needed to use the OSGeo4W version of libpng, not the MSYS2 version.

Usage:

OSGEO4W_ROOT_MSYS=/c/OSGeo4W \
./configure \
--libpng-config=mswindows/osgeo4w/libpng-config

The included libpng-config is version-agnostic as long as ${OSGEO4W_ROOT_MSYS}/include/libpng${PNG_DLL_VERSION}/png.h exists.

[HuidaeCho]

Maybe, adding support for libpng-config and creating mswindows/osgeo4w/libpng-config would be better...

[HuidaeCho]

BTW, is libpng-config included in most Linux distributions and macOS? Safe to remove --with-png-(includes|libs)?

[HuidaeCho]

HAVE_PNG_H is not defined for Ubuntu gcc?
https://github.com/OSGeo/grass/actions/runs/3610010032/jobs/6083548001#step:10:160

ERROR: write_image: unknown file type: .png

from

grass/lib/pngdriver/write.c

Lines 39 to 44 in e8f26dd

	#ifdef HAVE_PNG_H
	else if (G_strcasecmp(p, ".png") == 0)
	write_png();
	#endif
	else
	G_fatal_error("write_image: unknown file type: %s", p);

[HuidaeCho]

libpng-config from Ubuntu 20.04 seems to be broken. It doesn't print an -L flag for --ldflags.

...
libs="-lpng16"
...
L_opts=
R_opts=
...
ldopts=""
...
    --ldflags)
        echo ${ldopts} ${L_opts} ${R_opts} {$libs}

[nilason]

You need to change HAVE_PNG_H to HAVE_PNG in the files:

grass/lib/pngdriver/read.c

Line 33 in e8f26dd

#ifdef HAVE_PNG_H

grass/lib/pngdriver/write.c

Line 39 in e8f26dd

#ifdef HAVE_PNG_H

And then it works for me.

There is one difference to PNGLIB compared to before:

was -L/opt/local/lib -lpng -lz
with this: -L/opt/local/lib -lpng16

there is a missing -lz flag. I'm not sure of the importance or consequence of this.

[nilason]

HAVE_PNG_H is not defined for Ubuntu gcc?

HAVE_PNG_H was defined before with: LOC_CHECK_INCLUDES(png.h,PNG,$PNGINC), but not with this PR. HAVE_PNG serves the same purpose.

[HuidaeCho]

there is a missing -lz flag. I'm not sure of the importance or consequence of this.

Not sure either, but if -lz is missing from the system libpng-config, maybe harmless.

Changing HAVE_PNG to HAVE_PNG_H is more consistent with other *_H and safer (previously it was HAVE_PNG_H)

[nilason]

there is a missing -lz flag. I'm not sure of the importance or consequence of this.

Not sure either, but if -lz is missing from the system libpng-config, maybe harmless.

Well, it is there. Using libpng-config --static --ldflags I get -L/opt/local/lib -lpng16 -lz.

it comes from:
all_libs="-lpng16 -lz "
originating from libpng source file libpng-config.in:
all_libs="-lpng@PNGLIB_MAJOR@@PNGLIB_MINOR@ @LIBS@"

$ZLIB="-lz" and $MATHLIB="" (on my system) was previously set as mandatory (and added) by autoconf with:
LOC_CHECK_LIBS(png,png_read_image,PNG,$PNGLIB,PNGLIB,,$ZLIB $MATHLIB,)
see:

grass/aclocal.m4

Lines 145 to 159 in e8f26dd

	dnl $1 = library
	dnl $2 = function
	dnl $3 = descriptive name
	dnl $4 = LDFLAGS initialiser
	dnl $5 = result variable
	dnl $6 = mandatory dependencies (not added to $5)
	dnl $7 = mandatory dependencies (added to $5)
	dnl $8 = ACTION-IF-NOT-FOUND
	dnl $9+ = optional dependencies
	define(LOC_CHECK_LIBS_0,[
	AC_CHECK_LIB($1, $2, $5="$$5 -l$1 $7",[
	[$8]
	],$6 $7)
	])

Maybe libpng-config --static --ldflags could be used instead of libpng-config --ldflags?

Re: HAVE_PNG vs HAVE_PNG_H. There is already a bunch of similar to the former one with e.g. HAVE_OGR, HAVE_GDAL, HAVE_PDAL. But no big issue, works either way.

[HuidaeCho]

libpng-config --static --ldflags

That's interesting.. Only --static --ldflags includes -lm -lz. This is my output from Slackware:

-L/usr/lib64 -lpng16 -lm -lz -lm

ldd /usr/lib64/libpng16.so actually produces

        linux-vdso.so.1 (0x00007fff14bd0000)
        libz.so.1 => /lib64/libz.so.1 (0x00007f4860828000)
        libm.so.6 => /lib64/libm.so.6 (0x00007f48606e0000)
        libc.so.6 => /lib64/libc.so.6 (0x00007f4860501000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f48608ae000)

including libz.so.1 and libm.so.6, so I guess libpng-config --static --ldflags would be safer although we're not really doing "static linking".

[nilason]

so I guess libpng-config --static --ldflags would be safer although we're not really doing "static linking".

Indeed, that seems to be the safest way not to introduce regression.

[nilason]

Looks good to me!