Add cmake file

From mlabbe#99.

Author: CongliWANG

CMakeLists.txt

@@ -0,0 +1,40 @@
+cmake_minimum_required(VERSION 3.8.0)
+project(nativefiledialog)
+
+if(UNIX AND NOT APPLE AND NOT ANDROID)
+    set(LINUX 1)
+endif()
+
+set(SOURCES src/nfd_common.c)
+
+# Add specific implementations
+if (WIN32)
+    list(APPEND SOURCES src/nfd_win.cpp)
+    list(APPEND PREPROCESSOR_DEFINITIONS 
+        UNICODE 
+        _UNICODE
+    )
+elseif (APPLE)
+    list(APPEND SOURCES src/nfd_cocoa.m)
+elseif (LINUX)
+    list(APPEND SOURCES src/nfd_gtk.c)
+else()
+    message(FATAL_ERROR "Cannot detect your system")
+endif()
+
+add_library(nativefiledialog ${SOURCES})
+target_include_directories(nativefiledialog PUBLIC src/include)
+target_compile_definitions(nativefiledialog PUBLIC ${PREPROCESSOR_DEFINITIONS})
+
+if(LINUX)
+    find_package(PkgConfig)
+    if (PKG_CONFIG_FOUND)
+        pkg_check_modules(GTK "gtk+-3.0")
+        if (GTK_FOUND)    
+            target_link_libraries(nativefiledialog ${GTK_LIBRARIES})
+            add_definitions(${GTK_CFLAGS} ${GTK_CFLAGS_OTHER})
+        endif()
+    endif()
+endif()
+
+install(TARGETS nativefiledialog DESTINATION lib)