Do not call find_package(pybind11 QUIET) if IDYNTREE_USES_PYTHON_PYBIND11 is OFF

Calling find_package(pybind11) can create configuration failures even if the QUIET option is passed
(see #876). As at the moment there is not automatic
detection of pybind11 to enable the pybind11 bindings, it is just easier to properly search for pybind11
if IDYNTREE_USES_PYTHON_PYBIND11 is ON, and do not do anything otherwise.

Fix related to #876 .

Author: traversaro

bindings/CMakeLists.txt

@@ -97,13 +97,9 @@ endif()
 
 
 option(IDYNTREE_USES_PYTHON_PYBIND11 "Create iDynTree Python bindings using pybind11" FALSE)
-find_package(pybind11 QUIET)
 if (IDYNTREE_USES_PYTHON_PYBIND11)
-    if (${pybind11_FOUND})
-        add_subdirectory(pybind11)
-    else()
-       MESSAGE(FATAL_ERROR "pybind11 not found, impossible to generate iDynTree pybind11 bindings.")
-    endif()
+    find_package(pybind11 REQUIRED)
+    add_subdirectory(pybind11)
 endif()
 
 # Install main __init__.py file.