Gimp Build for Win

Follow the instructions
https://msys2.github.io/

Get some libraries script.
---------------------------------------------------------------------------------------
#!/bin/sh
# Installs runtime dependencies.
# User-oriented convenience script for MSYS2 installations.
# This shouldn't be used as part of anybody's build scripting.

case "$MSYSTEM" in
    MINGW64)
        ARCH=$ARCH
        ;;
    MINGW32)
        ARCH=i686
        ;;
    *)
        echo >&2 "ERROR: unsupported system."
        echo >&2 "This script must be run from either the MINGW32 or the MINGW64 "
        echo >&2 "environment of an MSYS2 installation on Windows."
        exit 2
        ;;

esac# Update packages caches

pacman -Sy --noconfirm

# Build and runtime deps, all in one go
pacman -S --needed --noconfirm \
	base-devel \
	git \
	scons \
	gtk-doc \
	mingw-w64-$ARCH-toolchain \
	mingw-w64-$ARCH-gtk2 \
	mingw-w64-$ARCH-json-c \
	mingw-w64-$ARCH-json-glib \
	mingw-w64-$ARCH-gexiv2 \
	mingw-w64-$ARCH-lcms2

pacman -S mingw-w64-$ARCH-ffmpeg # support for various formats

pacman -S mingw-w64-$ARCH-ghostscript # support for Ghostscript

pacman -S mingw-w64-$ARCH-libmng # support for MNG format

pacman -S mingw-w64-$ARCH-libwmf # support for WMF format

pacman -S mingw-w64-$ARCH-xpm-nox # support for XPM format

pacman -S mingw-w64-$ARCH-openexr # support for EXR format

pacman -S mingw-w64-$ARCH-libraw # support for RAW format

pacman -S mingw-w64-$ARCH-vala

pacman -S mingw-w64-$ARCH-python2 # for Python support

pacman -S mingw-w64-$ARCH-python2-pygtk # needed by python support

pacman -S mingw-w64-$ARCH-poppler

pacman -S mingw-w64-$ARCH-webkitgtk2 # for Webpage and html help support

--------------------------------------------------------------------------------------
edit your .bash-console -file and add lines

case "$MSYSTEM" in
	MINGW64)
		ARCH=x86_64 ; BASE=64
		;;
	MINGW32)
		ARCH=i686 ; BASE=32
		;;
	*)
		;;
esac

export PREFIX=/opt/gimp$BASE  # or the path you like to use..
export PATH=$PREFIX/bin:$PATH
export PKG_CONFIG_PATH=$PREFIX/lib/pkgconfig:$PKG_CONFIG_PATH
export MAKEFLAGS=-j4 # multicore support
export CPPFLAGS='-I'$PREFIX'/include -I/mingw'$BASE'/'$ARCH'-w64-mingw32/include -I/mingw'$BASE'/include/python2.7'

export CPATH="$PREFIX'/include:/mingw'$BASE'/'$ARCH'-w64-mingw32/include:/mingw'$BASE'/include/python2.7"
export ACLOCAL_AMFLAGS=-I/mingw'$BASE'/share/aclocal

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Updating core libraries is best done with Msys2 shell and building projects must be done with launching mingw32_shell.bat or mingw64_shell.bat.
Remember to launch those shells with 'run as administrator', otherwise there might be insufficient privileges at some point of build processes.

So open your mingw32 or mingw64 shell as administrator and copy paste these lines

(building babl from git)

git clone https://git.gnome.org/browse/babl
cd babl
./autogen.sh --prefix=$PREFIX
make
make install
cd ..

(building gegl from git)

git clone https://git.gnome.org/browse/gegl
cd gegl
./autogen.sh --prefix=$PREFIX
make
make install
cd ..

(next we need libmypaint for the new brush engines. It doesnt have autogen.sh bust uses SCons as build tool)
git clone https://github.com/mypaint/libmypaint.git
cd libmypaint
scons prefix=$PREFIX
scons prefix=$PREFIX install
cd ..


(and lastly building gimp)
git clone https://git.gnome.org/browse/gimp
cd gimp
./autogen.sh --prefix=$PREFIX
make
make install
cd ..