migrate most SYSTEM_* config from set_soenv.in to config_host.mk.in

should be pretty safe.
The main risk if is something rely on the 'undef if empty' behavior of
the typical trap is the use of *= in dmake
a git grep of "\$(SYSTEM_" did not reveal any problematic case.

config_host.mk.in

@@ -3,3 +3,63 @@
  # or to be sourced in a shell session
  #
 export SRC_ROOT="@SRC_ROOT@"
+export SYSTEM_APACHE_COMMONS="@SYSTEM_APACHE_COMMONS@"
+export SYSTEM_BOOST="@SYSTEM_BOOST@"
+export SYSTEM_BSH="@SYSTEM_BSH@"
+export SYSTEM_CAIRO="@SYSTEM_CAIRO@"
+export SYSTEM_CPPUNIT="@SYSTEM_CPPUNIT@"
+export SYSTEM_CURL="@SYSTEM_CURL@"
+export SYSTEM_DB="@SYSTEM_DB@"
+export SYSTEM_DB_CFLAGS="@SYSTEM_DB_CFLAGS@"
+export SYSTEM_DICTS="@SYSTEM_DICTS@"
+export SYSTEM_EXPAT="@SYSTEM_EXPAT@"
+export SYSTEM_GDKPIXBUF="@SYSTEM_GDKPIXBUF@"
+export SYSTEM_GENBRK="@SYSTEM_GENBRK@"
+export SYSTEM_GENCCODE="@SYSTEM_GENCCODE@"
+export SYSTEM_GENCMN="@SYSTEM_GENCMN@"
+export SYSTEM_GETTEXT="@SYSTEM_GETTEXT@"
+export SYSTEM_GLIB="@SYSTEM_GLIB@"
+export SYSTEM_GRAPHITE="@SYSTEM_GRAPHITE@"
+export SYSTEM_HSQLDB="@SYSTEM_HSQLDB@"
+export SYSTEM_HUNSPELL="@SYSTEM_HUNSPELL@"
+export SYSTEM_HYPH="@SYSTEM_HYPH@"
+export SYSTEM_ICU="@SYSTEM_ICU@"
+export SYSTEM_JFREEREPORT="@SYSTEM_JFREEREPORT@"
+export SYSTEM_JPEG="@SYSTEM_JPEG@"
+export SYSTEM_LIBC="@SYSTEM_LIBC@"
+export SYSTEM_LIBCDR="@SYSTEM_LIBCDR@"
+export SYSTEM_LIBCMIS="@SYSTEM_LIBCMIS@"
+export SYSTEM_LIBCROCO="@SYSTEM_LIBCROCO@"
+export SYSTEM_LIBEXTTEXTCAT="@SYSTEM_LIBEXTTEXTCAT@"
+export SYSTEM_LIBEXTTEXTCAT_DATA="@SYSTEM_LIBEXTTEXTCAT_DATA@"
+export SYSTEM_LIBGSF="@SYSTEM_LIBGSF@"
+export SYSTEM_LIBJPEG="@SYSTEM_LIBJPEG@"
+export SYSTEM_LIBPNG="@SYSTEM_LIBPNG@"
+export SYSTEM_LIBRSVG="@SYSTEM_LIBRSVG@"
+export SYSTEM_LIBVISIO="@SYSTEM_LIBVISIO@"
+export SYSTEM_LIBWPD="@SYSTEM_LIBWPD@"
+export SYSTEM_LIBWPG="@SYSTEM_LIBWPG@"
+export SYSTEM_LIBWPS="@SYSTEM_LIBWPS@"
+export SYSTEM_LIBXML="@SYSTEM_LIBXML@"
+export SYSTEM_LPSOLVE="@SYSTEM_LPSOLVE@"
+export SYSTEM_LUCENE="@SYSTEM_LUCENE@"
+export SYSTEM_MDDS="@SYSTEM_MDDS@"
+export SYSTEM_MESA_HEADERS="@SYSTEM_MESA_HEADERS@"
+export SYSTEM_MYSQL="@SYSTEM_MYSQL@"
+export SYSTEM_MYSQL_CPPCONN="@SYSTEM_MYSQL_CPPCONN@"
+export SYSTEM_MYTHES="@SYSTEM_MYTHES@"
+export SYSTEM_NEON="@SYSTEM_NEON@"
+export SYSTEM_NSS="@SYSTEM_NSS@"
+export SYSTEM_ODBC_HEADERS="@SYSTEM_ODBC_HEADERS@"
+export SYSTEM_OPENSSL="@SYSTEM_OPENSSL@"
+export SYSTEM_PANGO="@SYSTEM_PANGO@"
+export SYSTEM_POPPLER="@SYSTEM_POPPLER@"
+export SYSTEM_POSTGRESQL="@SYSTEM_POSTGRESQL@"
+export SYSTEM_REDLAND="@SYSTEM_REDLAND@"
+export SYSTEM_SAMPLEICC="@SYSTEM_SAMPLEICC@"
+export SYSTEM_SANE_HEADER="@SYSTEM_SANE_HEADER@"
+export SYSTEM_SAXON="@SYSTEM_SAXON@"
+export SYSTEM_SERVLETAPI="@SYSTEM_SERVLETAPI@"
+export SYSTEM_STDLIBS="@SYSTEM_STDLIBS@"
+export SYSTEM_VIGRA="@SYSTEM_VIGRA@"
+export SYSTEM_ZLIB="@SYSTEM_ZLIB@"