* builds/unix/configure.raw: Restore `SYSTEM_ZLIB` variable.

This was accidentally removed with commit 93ebcbd0 almost eight years ago.

builds/unix/configure.raw

@@ -317,6 +317,12 @@ if test x"$with_zlib" = xyes -a "$have_zlib" = no; then
   AC_MSG_ERROR([external zlib support requested but library not found])
 fi
 
+SYSTEM_ZLIB=
+if test "$have_zlib" != no; then
+  SYSTEM_ZLIB=yes
+fi
+AC_SUBST([SYSTEM_ZLIB])
+
 
 # check for system libbz2
 

devel/ftoption.h

@@ -219,6 +219,10 @@ FT_BEGIN_HEADER
    *   If you use a build system like cmake or the `configure` script,
    *   options set by those programs have precedence, overwriting the value
    *   here with the configured one.
+   *
+   *   If you use the GNU make build system directly (that is, without the
+   *   `configure` script) and you define this macro, you also have to pass
+   *   `SYSTEM_ZLIB=yes` as an argument to make.
    */
 /* #define FT_CONFIG_OPTION_SYSTEM_ZLIB */
 

include/freetype/config/ftoption.h

@@ -219,6 +219,10 @@ FT_BEGIN_HEADER
    *   If you use a build system like cmake or the `configure` script,
    *   options set by those programs have precedence, overwriting the value
    *   here with the configured one.
+   *
+   *   If you use the GNU make build system directly (that is, without the
+   *   `configure` script) and you define this macro, you also have to pass
+   *   `SYSTEM_ZLIB=yes` as an argument to make.
    */
 /* #define FT_CONFIG_OPTION_SYSTEM_ZLIB */