cmake/utils.cmake doesn't properly quote arguments to "ar"

When utils.cmake builds a directive file for "ar" as part of building libshaderc/libshaderc_combined.a, it fails to properly quote the arguments to "ar", thus causing "ar" to fail if the build directory happens to contain any characters which might be construed to mean something special on the command line. Here's an example from me trying to build a debian package:

```
[313/319] cd "/build/shaderc-2018.0~dev+20/debian/build/libshaderc" && /usr/bin/ar -M <
shaderc_combined.ar
FAILED: libshaderc/libshaderc_combined.a
cd "/build/shaderc-2018.0~dev+20/debian/build/libshaderc" && /usr/bin/ar -M < shaderc_combined.ar
/usr/bin/ar: /build/shaderc-2018.0~dev: No such file or directory
+Syntax error in archive script, line 1

```
Here is a patch to fix the problem:

```
diff --git a/src/cmake/utils.cmake b/src/cmake/utils.cmake
index ed3c733..519d28f 100644
--- a/src/cmake/utils.cmake
+++ b/src/cmake/utils.cmake
@@ -186,9 +186,9 @@ function(shaderc_combine_static_lib new_target target)
DEPENDS ${all_libs}
COMMAND libtool -static -o ${libname} ${lib_target_list})
else()
- string(REPLACE ";" "> \naddlib $ '\naddlib '$")
+ "create '${libname}'\naddlib '$'")
set(build_script_file "${start_of_file}\nsave\nend\n")

file(GENERATE OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/${new_target}.ar"

```