Using system miniz in Linuxes.

Also enabling miniz large file support on gnu compilers.

src/miniz/CMakeLists.txt

@@ -3,7 +3,7 @@ cmake_minimum_required(VERSION 2.6)
 
 add_library(miniz INTERFACE)
 
-if(NOT SLIC3R_STATIC)
+if(NOT SLIC3R_STATIC OR CMAKE_SYSTEM_NAME STREQUAL "Linux")
     find_package(miniz 2.1 QUIET)
 endif() 
 
@@ -19,6 +19,10 @@ else()
         miniz.h
     )
     
+    if("${CMAKE_C_COMPILER_ID}" STREQUAL "GNU")
+        target_compile_definitions(miniz_static PRIVATE _GNU_SOURCE)
+    endif()
+
     target_link_libraries(miniz INTERFACE miniz_static)
     target_include_directories(miniz INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})