remove linking to libpython on linux/OSX (#703)

* remove linking to libpython on linux

* add OSX libpython free building

* add automatic discovery of include python dir back in

* make the libs non required for building on manylinux

dlib/cmake_utils/add_python_module

@@ -69,10 +69,10 @@ if (PYTHON3)
         FIND_PACKAGE(Boost 1.41.0 COMPONENTS python)
     endif()
     set(Python_ADDITIONAL_VERSIONS 3.5 3.6)
-    FIND_PACKAGE(PythonLibs 3.4 REQUIRED)
+    FIND_PACKAGE(PythonLibs 3.4)
 else()
     FIND_PACKAGE(Boost 1.41.0 COMPONENTS python)
-    FIND_PACKAGE(PythonLibs 2.6 REQUIRED)
+    FIND_PACKAGE(PythonLibs 2.6)
 endif()
 
 if (NOT Boost_FOUND)
@@ -126,7 +126,6 @@ else()
 endif()
 
 message(STATUS "USING BOOST_LIBS: ${Boost_LIBRARIES}")
-message(STATUS "USING PYTHON_LIBS: ${PYTHON_LIBRARIES}")
 
 if (CMAKE_COMPILER_IS_GNUCXX)
     # Just setting CMAKE_POSITION_INDEPENDENT_CODE should be enough to set
@@ -148,7 +147,8 @@ include(${CMAKE_CURRENT_LIST_DIR}/../cmake)
 # output name is set to what the user asked for (i.e. no _).
 macro(add_python_module module_name module_sources )
    ADD_LIBRARY(${module_name}_ SHARED ${module_sources} ${ARGN} )
-   TARGET_LINK_LIBRARIES(${module_name}_ ${Boost_LIBRARIES} ${PYTHON_LIBRARIES}  dlib::dlib)
+   TARGET_LINK_LIBRARIES(${module_name}_ ${Boost_LIBRARIES} dlib::dlib)
+
    if(WIN32 AND NOT CYGWIN)
       SET_TARGET_PROPERTIES( ${module_name}_
          PROPERTIES
@@ -163,9 +163,18 @@ macro(add_python_module module_name module_sources )
          SUFFIX ".dll"
          OUTPUT_NAME ${module_name}
          )
+    elseif(APPLE)
+      SET_TARGET_PROPERTIES( ${module_name}_
+         PROPERTIES
+         LINK_FLAGS "-undefined dynamic_lookup"
+         PREFIX ""
+         SUFFIX ".so"
+         OUTPUT_NAME ${module_name}
+         )
    else()
       SET_TARGET_PROPERTIES( ${module_name}_
          PROPERTIES
+         LINK_FLAGS "-shared"
          PREFIX ""
          SUFFIX ".so"
          OUTPUT_NAME ${module_name}