Make Ghostscript build on ppc64el and arm64

The libpng shipping with Ghostscript does not build on ppc64el and
arm64 without disabling certain optimizations. For ppc64el this is
already solved in the Ghostscript GIT and part of the upcoming 9.53:

http://git.ghostscript.com/?p=ghostpdl.git;a=commitdiff;h=1c9bd8e0aa4
https://bugs.ghostscript.com/show_bug.cgi?id=702236

For arm64 there is a similar problem but Ghostscript developers seem
not yet aware of it. A hint for a solution can be found in another
project though:

imagemin/optipng-bin#97

patches/ghostscript-disable-ppc-optimisations-in-libpng.patch

@@ -0,0 +1,45 @@
+From: Chris Liddell <chris.liddell@artifex.com>
+Date: Tue, 24 Mar 2020 15:19:23 +0000 (+0000)
+Subject: Disable PPC optimisations in libpng
+X-Git-Tag: ghostpdl-9.52-test-base-1~2
+X-Git-Url: http://git.ghostscript.com/?p=ghostpdl.git;a=commitdiff_plain;h=1c9bd8e0aa4
+
+Disable PPC optimisations in libpng
+
+Since we don't include the libpng source file with the PowerPC optimizations
+in our build, and it only applies to reading, which we don't rely on, just
+disable calling the optimizations.
+
+Cures a linker error on PowerPCle platforms
+---
+
+diff --git a/configure.ac b/configure.ac
+index 638c56641..5be1eac6a 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -1252,7 +1252,24 @@ else
+           SHARE_LIBPNG=0
+           LIBPNGDIR=$srcdir/libpng
+           PNGDEVS="$PNGDEVS_ALL"
+-  else
++          case $host in
++            *powerpc*)
++                # Building on PPC with gcc, disable libpng's PPC specific
++                # optimizations: we don't include the source file for it
++                # and it only applies to reading, which we don't use.
++                if test $GCC = yes; then
++                  CFLAGS="$CFLAGS -DPNG_POWERPC_VSX_OPT=0"
++                fi
++            ;;
++            *arm64*|*aarch64*)
++                # Building on arm64 with gcc, disable libpng's neon
++                # optimizations.
++                if test $GCC = yes; then
++                  CFLAGS="$CFLAGS -DPNG_ARM_NEON_OPT=0"
++                fi
++            ;;
++          esac
++ else
+           AC_MSG_RESULT([no])
+           AC_CHECK_LIB(png, png_create_write_struct, [
+             AC_CHECK_HEADERS(png.h, [

snapcraft.yaml

@@ -115,7 +115,7 @@ apps:
     plugs: [home, system-config]
 
 parts:
-  cups-patches:
+  patches:
     plugin: dump
     source: patches
     organize:
@@ -202,7 +202,7 @@ parts:
         - -usr/share/doc
         - -usr/share/doc-base
         - -usr/share/lintian
-    after: [cups-patches]
+    after: [patches]
   qpdf:
     source: https://github.com/qpdf/qpdf/releases/download/release-qpdf-10.0.1/qpdf-10.0.1.tar.gz
     plugin: autotools
@@ -228,6 +228,11 @@ parts:
   ghostscript:
     source: https://github.com/ArtifexSoftware/ghostpdl-downloads/releases/download/gs952/ghostscript-9.52.tar.gz
     plugin: autotools
+    override-build: |
+        set -eux
+        patch -p1 < $SNAPCRAFT_STAGE/patches/ghostscript-disable-ppc-optimisations-in-libpng.patch
+        ./autogen.sh
+        snapcraftctl build
     configflags:
         - --without-x
         - --disable-gtk
@@ -259,7 +264,7 @@ parts:
         - -snap/cups
     organize:
       snap/cups/current/share: share
-    after: [cups]
+    after: [patches, cups]
   cups-filters:
     source: http://www.openprinting.org/download/cups-filters/cups-filters-1.27.5.tar.xz
     plugin: autotools