Only need libunwind for testing (#2093)

### Description of changes: 
* Libunwind is only needed when building tests.

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under the terms of the Apache 2.0 license and the ISC license.

CMakeLists.txt

@@ -145,7 +145,7 @@ if(NOT DISABLE_PERL)
   find_package(Perl REQUIRED)
 endif()
 
-if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux" AND NOT CMAKE_CROSSCOMPILING)
+if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux" AND BUILD_TESTING AND NOT CMAKE_CROSSCOMPILING)
   find_package(PkgConfig QUIET)
   if (PkgConfig_FOUND)
     pkg_check_modules(LIBUNWIND libunwind-generic)