Trac #29424: Slackware: system packages fix

Building git for `slackware-14.2-minimal`
(https://github.com/mkoeppe/sage/runs/542501916):
{{{
mkdir -p po/build/locale/vi/LC_MESSAGES/ && msgfmt --check --statistics
-o po/build/locale/vi/LC_MESSAGES/git.mo po/vi.po
msgfmt: error while loading shared libraries: libcroco-0.6.so.3: cannot
open shared object file: No such file or directory
Makefile:2150: recipe for target 'po/build/locale/vi/LC_MESSAGES/git.mo'
failed
make[4]: *** [po/build/locale/vi/LC_MESSAGES/git.mo] Error 127
}}}

libpng not found:
{{{
configure:12817: checking for libpng >= 1.2
configure:12824: $PKG_CONFIG --exists --print-errors "libpng >= 1.2"
Package libpng was not found in the pkg-config search path.
Perhaps you should add the directory containing `libpng.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libpng' found
}}}

URL: https://trac.sagemath.org/29424
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Dima Pasechnik

Author: Release Manager

build/pkgs/libgd/distros/slackware.txt

@@ -1 +1,3 @@
 gd
+# shared library dependencies of gd
+fontconfig libXpm libX11 libxcb libXau libXdmcp

build/pkgs/libpng/distros/slackware.txt

@@ -0,0 +1 @@
+libpng

build/pkgs/slackware-bootstrap.txt

@@ -1,6 +1,4 @@
-# Packages needed for ./bootstrap
-gettext-tools
+# Additional packages needed for ./bootstrap
 autoconf
 automake
 libtool
-pkg-config

build/pkgs/slackware.txt

@@ -11,3 +11,7 @@ python-2.7
 flex
 # for https upstream_url downloads
 ca-certificates
+pkg-config
+gettext-tools
+libcroco         # gettext dependency (msgfmt)
+libxml2