Add libmagic wrapper

Also builds separate static library linuxdeploy_util providing old util
header and new magic wrapper.

src/util/CMakeLists.txt

@@ -0,0 +1,2 @@
+add_library(linuxdeploy_util STATIC magicwrapper.cpp magicwrapper.h util.h)
+target_include_directories(linuxdeploy_util PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})

src/util/magicwrapper.cpp

@@ -0,0 +1,49 @@
+// system includes
+#include <magic.h>
+#include <string>
+
+// local includes
+#include "magicwrapper.h"
+
+namespace linuxdeploy {
+    namespace util {
+        namespace magic {
+            class MagicWrapper::PrivateData {
+                private:
+                    magic_t cookie;
+
+                public:
+                    PrivateData() noexcept(false) {
+                        cookie = magic_open(MAGIC_DEBUG | MAGIC_SYMLINK | MAGIC_MIME_TYPE);
+
+                        if (cookie == nullptr)
+                            throw MagicError("Failed to open magic database");
+                    }
+
+                    ~PrivateData() {
+                        if (cookie != nullptr) {
+                            magic_close(cookie);
+                            cookie = nullptr;
+                        }
+                    };
+
+                    std::string fileType(const std::string& path) {
+                        const auto* buf = magic_file(cookie, path.c_str());
+
+                        if (buf == nullptr)
+                            return "";
+
+                        return buf;
+                    }
+            };
+
+            MagicWrapper::MagicWrapper() {
+                d = new PrivateData();
+            }
+
+            MagicWrapper::~MagicWrapper() {
+                delete d;
+            }
+        }
+    }
+}

src/util/magicwrapper.h

@@ -0,0 +1,37 @@
+// system includes
+#include <exception>
+#include <string>
+#include <utility>
+
+#pragma once
+
+namespace linuxdeploy {
+    namespace util {
+        namespace magic {
+            // thrown by constructors if opening the magic database fails
+            class MagicError : std::exception {
+                private:
+                    const std::string message;
+
+                public:
+                    explicit MagicError() : message() {}
+
+                    explicit MagicError(std::string message) : message(std::move(message)) {}
+
+                    const char* what() {
+                        return message.c_str();
+                    }
+            };
+
+            class MagicWrapper {
+                private:
+                    class PrivateData;
+                    PrivateData *d;
+
+                public:
+                    MagicWrapper();
+                    ~MagicWrapper();
+            };
+        }
+    }
+}