Add libmagic wrapper

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

cmake/Modules/FindLibMagic.cmake

@@ -0,0 +1,21 @@
+message(STATUS "Searching for libmagic")
+
+find_library(LIBMAGIC_A libmagic.a)
+
+if(NOT LIBMAGIC_A)
+    message(FATAL_ERROR "libmagic.a not found")
+endif()
+
+find_path(LIBMAGIC_MAGIC_H
+    NAMES magic.h
+    HINTS ${CMAKE_INSTALL_PREFIX}
+    PATH_SUFFIXES include include/linux
+    )
+
+if(NOT LIBMAGIC_MAGIC_H)
+    message(FATAL_ERROR "magic.h not found")
+endif()
+
+add_library(libmagic_static INTERFACE IMPORTED)
+set_property(TARGET libmagic_static PROPERTY INTERFACE_INCLUDE_DIRECTORIES ${LIBMAGIC_MAGIC_H})
+set_property(TARGET libmagic_static PROPERTY INTERFACE_LINK_LIBRARIES ${LIBMAGIC_A})