[gn build] Add a flag zlib_path to provide the path to zlib on Windows.

Also, automatically set llvm_enable_zlib to true when zlib_path is set.

Differential Revision: https://reviews.llvm.org/D80042

Author: sebmarchand

llvm/utils/gn/build/libs/zlib/BUILD.gn

@@ -2,7 +2,13 @@ import("//llvm/utils/gn/build/libs/zlib/enable.gni")
 
 config("zlib_config") {
   visibility = [ ":zlib" ]
-  libs = [ "z" ]
+  if (host_os == "win") {
+    assert(zlib_path != "", "Please provide the path to zlib")
+    include_dirs = [ zlib_path ]
+    libs = [ "$zlib_path/zlib.lib" ]
+  } else {
+    libs = [ "z" ]
+  }
 }
 
 group("zlib") {

llvm/utils/gn/build/libs/zlib/enable.gni

@@ -1,4 +1,11 @@
+declare_args() {
+  if (host_os == "win") {
+    # On Windows, path to a directory containing zlib headers and zlib.lib.
+    zlib_path = ""
+  }
+}
+
 declare_args() {
   # Whether to include code that links against zlib.
-  llvm_enable_zlib = host_os != "win"
+  llvm_enable_zlib = host_os != "win" || zlib_path != ""
 }