configure: Support for libpng-config (#2679)

* configure: PNGVER environment variable to pickup libpng16.dll from OSGeo4W v2

* Add libpng-config (part of libpng) support and missing script for OSGeo4W

* Add --dll-version to libpng-config; Update gdal-config to detect libpng version

* Add --libpng-config to build_centos.sh

* Fix for clang (no implicit declaration of function)

* Use libpng-config --ldflags instead of --libs to include -L flags

* Let's try LDFLAGS for Ubuntu 20.04

* Define HAVE_PNG_H

* Consistent comments

* Delete trailing spaces; Replace tabs+spaces+tabs with tabs+spaces

* Copyright year to 2022

* Revert build_ubuntu-20.04.sh

* Add --static flag to libpng-config to include -lm -lz

Co-authored-by: Huidae Cho <grass4u@gmail.com>

.github/workflows/build_centos.sh

@@ -33,6 +33,7 @@ export CXXFLAGS="-std=gnu++11"
     --with-cairo --with-cairo-ldflags=-lfontconfig \
     --with-freetype --with-freetype-includes=/usr/include/freetype2 \
     --with-proj --with-proj-share=/usr/share/proj \
+    --with-libpng=/usr/bin/libpng-config \
     --with-gdal=/usr/bin/gdal-config \
     --without-zstd \
     --without-tiff \