Find external system libraries for ebuild.

Updated from 3.0 version of patch to apply against 4.0.

--- apitrace/CMakeLists.txt
+++ apitrace/CMakeLists.txt
@@ -224,23 +224,14 @@ set (WRAPPER_INSTALL_DIR ${LIB_ARCH_INSTALL_DIR}/wrappers)
 # - on unices to prevent symbol collisions when tracing applications that link
 # against other versions of these libraries
 
-set (ZLIB_INCLUDE_DIRS ${CMAKE_CURRENT_SOURCE_DIR}/thirdparty/zlib)
-set (ZLIB_LIBRARIES z_bundled)
-add_subdirectory (thirdparty/zlib)
-
-include_directories (${ZLIB_INCLUDE_DIRS})
-
-set (SNAPPY_INCLUDE_DIRS ${CMAKE_CURRENT_SOURCE_DIR}/thirdparty/snappy)
-set (SNAPPY_LIBRARIES snappy_bundled)
-add_subdirectory (thirdparty/snappy)
+find_package (ZLIB REQUIRED)
+include_directories (${ZLIB_INCLUDE_DIRS})
 
+find_package (SNAPPY REQUIRED)
 include_directories (${SNAPPY_INCLUDE_DIRS})
 
-set (PNG_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/thirdparty/libpng)
-set (PNG_DEFINITIONS "")
-set (PNG_LIBRARIES png_bundled)
-
-add_subdirectory (thirdparty/libpng)
+find_package (PNG REQUIRED)
+include_directories (${PNG_INCLUDE_DIRS})
 
 if (MSVC)
     add_subdirectory (thirdparty/getopt)
diff --git a/cmake/FindSNAPPY.cmake b/cmake/FindSNAPPY.cmake
new file mode 100644
index 0000000..5cd64c3
--- /dev/null
+++ apitrace/cmake/FindSNAPPY.cmake
@@ -0,0 +1,31 @@
+# Find SNAPPY - A fast compressor/decompressor
+#
+# This module defines
+#  SNAPPY_FOUND - whether the qsjon library was found
+#  SNAPPY_LIBRARIES - the snappy library
+#  SNAPPY_INCLUDE_DIR - the include path of the snappy library
+#
+
+if (SNAPPY_INCLUDE_DIR AND SNAPPY_LIBRARIES)
+
+  # Already in cache
+  set (SNAPPY_FOUND TRUE)
+
+else (SNAPPY_INCLUDE_DIR AND SNAPPY_LIBRARIES)
+
+  find_library (SNAPPY_LIBRARIES
+    NAMES
+    snappy
+    PATHS
+  )
+
+  find_path (SNAPPY_INCLUDE_DIR
+    NAMES
+    snappy.h
+    PATHS
+  )
+
+  include(FindPackageHandleStandardArgs)
+  find_package_handle_standard_args(SNAPPY DEFAULT_MSG SNAPPY_LIBRARIES SNAPPY_INCLUDE_DIR)
+
+endif (SNAPPY_INCLUDE_DIR AND SNAPPY_LIBRARIES)