Link to CMake Python3::Module target (#98)

Since ecflow is built as a `module` (i.e. a shared library, dynamically loaded by the Python interpreter), the linkage in macOs requires passing flags `-undefined` and `-dynamic_lookup`.
This is handled directly by CMake which, since CMake 3.15, defines a specific target `Python3::Module` to be used when building Python modules.

Re ECFLOW-1950

Author: marcosbento

CMakeLists.txt

@@ -58,7 +58,6 @@ option( ENABLE_UI_BACKTRACE        "Print a UI debug backtrace"         OFF )
 option( ENABLE_UI_USAGE_LOG        "Enable UI usage logging"            OFF )
 option( ENABLE_SSL                 "Enable SSL encrypted communication" ON )
 option( ENABLE_PYTHON_PTR_REGISTER "Some compilers/boost versions do not register shared ptr automatically" OFF  )
-option( ENABLE_PYTHON_UNDEF_LOOKUP "Some boost/python versions are too closely linked" OFF  )
 option( ENABLE_HTTP                "Enable HTTP server (experimental)" ON  )
 option( ENABLE_HTTP_COMPRESSION    "Enable compression support by HTTP server" ON )
 option( ENABLE_UDP                 "Enable UDP server (experimental)" ON )
@@ -192,7 +191,6 @@ endif()
 ecbuild_info( "ENABLE_SERVER              : ${ENABLE_SERVER}" )
 ecbuild_info( "ENABLE_PYTHON              : ${ENABLE_PYTHON}" )
 ecbuild_info( "ENABLE_PYTHON_PTR_REGISTER : ${ENABLE_PYTHON_PTR_REGISTER}" )
-ecbuild_info( "ENABLE_PYTHON_UNDEF_LOOKUP : ${ENABLE_PYTHON_UNDEF_LOOKUP}" )
 ecbuild_info( "ENABLE_UI                  : ${ENABLE_UI}" )
 ecbuild_info( "ENABLE_TESTS               : ${ENABLE_TESTS} *if* disabled no need for boost test libs" )
 ecbuild_info( "ENABLE_ALL_TESTS           : ${ENABLE_ALL_TESTS}" )

Pyext/python3/CMakeLists.txt

@@ -8,16 +8,16 @@
 # nor does it submit to any jurisdiction.
 #
 
-# ENABLE_PYTHON_UNDEF_LOOKUP
-# on some systems (e.g. conda on macOS) the boost Python libraries seem to be too
-# closely linked with the exact version of the Python development libraries,
-# meaning that we get segfaults when we try to run a Python example. By removing
-# the Python libraries from the link line and adding -undefined dynamic_lookup
-# we can make our ecflow shared library more movable bewteen systems
-
-if (ENABLE_PYTHON_UNDEF_LOOKUP)
-  add_link_options(-undefined dynamic_lookup)
-endif()
+#
+#   Important!
+#
+# Notice that ecflow Python module is linked with `Python3::Module` CMake imported target,
+# to enable a loose linkage with Python development libraries -- this is necessary in some
+# platforms, such as conda-forge+macOS.
+#
+# In practice, this implies that `-undefined` and `-dynamic_lookup` linkage flags are used
+# and the Python development libraries are not linked directly into the ecflow Python module.
+#
 
 ecbuild_add_library(
   TARGET
@@ -35,7 +35,7 @@ ecbuild_add_library(
     node
     attributes
     core
-    Python3::Python
+    Python3::Module
     Boost::${ECFLOW_BOOST_PYTHON_COMPONENT}
     $<$<BOOL:${OPENSSL_FOUND}>:OpenSSL::SSL>
   CXXFLAGS