builder: msys2 fixes

Build zlib before ccache so that ccache does not try to use its bundled
zlib, which does not compile cleanly on cygwin/msys2.

A couple of related PRs for ccache:

ccache/ccache#311
ccache/ccache#310

Replace the usage of xzcat/bzcat in archive unpacking with `xz -dc` and
`bzip2 -dc` because @ZachBacon had some issues with xzcat trying to
compress instead of decompress.

When switching msys2 build modes, unset related env vars first,
sometimes they can get mixed up for some reason.

tools/builder/core.sh

@@ -114,11 +114,11 @@ DISTS=$DISTS'
     bzip2           https://github.com/nemequ/bzip2/releases/download/v1.0.6/bzip2-1.0.6.tar.gz                 lib/libbz2.a
     xz              https://tukaani.org/xz/xz-5.2.3.tar.gz                                                      lib/liblzma.a
     unzip           https://downloads.sourceforge.net/project/infozip/UnZip%206.x%20%28latest%29/UnZip%206.0/unzip60.tar.gz     bin/unzip
+    zlib            https://zlib.net/zlib-1.2.11.tar.gz                                                         lib/libz.a
     ccache          https://www.samba.org/ftp/ccache/ccache-3.4.3.tar.xz                                        bin/ccache
     zip             https://downloads.sourceforge.net/project/infozip/Zip%203.x%20%28latest%29/3.0/zip30.tar.gz                 bin/zip
     openssl         https://www.openssl.org/source/openssl-1.0.2o.tar.gz                                        lib/libssl.a
     cmake           https://cmake.org/files/v3.10/cmake-3.10.3.tar.gz                                           bin/cmake
-    zlib            https://zlib.net/zlib-1.2.11.tar.gz                                                         lib/libz.a
     m4              http://ftp.gnu.org/gnu/m4/m4-1.4.18.tar.xz                                                  bin/m4
     autoconf        https://ftp.gnu.org/gnu/autoconf/autoconf-2.69.tar.xz                                       bin/autoconf
     autoconf-archive http://mirror.team-cymru.org/gnu/autoconf-archive/autoconf-archive-2017.09.28.tar.xz       share/aclocal/ax_check_gl.m4
@@ -1059,10 +1059,10 @@ unpack_dist() {
             $TAR $@ -zxf "$dist_file"
             ;;
         *.tar.xz)
-            xzcat "$dist_file" | $TAR $@ -xf -
+            xz -dc "$dist_file" | $TAR $@ -xf -
             ;;
         *.tar.bz2)
-            bzcat "$dist_file" | $TAR $@ -xf -
+            bzip2 -dc "$dist_file" | $TAR $@ -xf -
             ;;
         *.zip)
             unzip -q "$dist_file"

tools/win/msys2-builder

@@ -6,6 +6,7 @@ case "$1" in
     -32)
         shift
         if [ "$MSYSTEM" != MINGW32 ]; then
+            unset MSYSTEM_PREFIX MSYSTEM_CARCH MSYSTEM_CHOST MINGW_CHOST MINGW_PREFIX MINGW_PACKAGE_PREFIX CONFIG_SITE ACLOCAL_PATH PKG_CONFIG_PATH
             OPWD=$PWD
             export MSYSTEM=MINGW32
             . /etc/profile || :
@@ -15,6 +16,7 @@ case "$1" in
     -64)
         shift
         if [ "$MSYSTEM" != MINGW64 ]; then
+            unset MSYSTEM_PREFIX MSYSTEM_CARCH MSYSTEM_CHOST MINGW_CHOST MINGW_PREFIX MINGW_PACKAGE_PREFIX CONFIG_SITE ACLOCAL_PATH PKG_CONFIG_PATH
             OPWD=$PWD
             export MSYSTEM=MINGW64
             . /etc/profile || :
@@ -105,6 +107,8 @@ DIST_TAR_ARGS="
 set_host_env() {
     ln -sf "$BUILD_ROOT/host" "$BUILD_ROOT/root"
 
+    unset MSYSTEM_PREFIX MSYSTEM_CARCH MSYSTEM_CHOST MINGW_CHOST MINGW_PREFIX MINGW_PACKAGE_PREFIX CONFIG_SITE ACLOCAL_PATH PKG_CONFIG_PATH
+
     OPWD=$PWD
     MSYSTEM=MSYS . /etc/profile || :
     eval "$BUILD_ENV"
@@ -114,6 +118,8 @@ set_host_env() {
 unset_host_env() {
     ln -sf "$BUILD_ROOT/target" "$BUILD_ROOT/root"
 
+    unset MSYSTEM_PREFIX MSYSTEM_CARCH MSYSTEM_CHOST MINGW_CHOST MINGW_PREFIX MINGW_PACKAGE_PREFIX CONFIG_SITE ACLOCAL_PATH PKG_CONFIG_PATH
+
     OPWD=$PWD
     MSYSTEM=$OMSYSTEM . /etc/profile || :
     eval "$BUILD_ENV"