FindPythonLibs: Find consistent Python interp, headers, libs (#13794)

When possible, get consistent version of the Python interpreter, headers path,
and library.

Now find_package(PythonLibs) internally calls find_package(PythonInterp
QUIET) and uses the resulting PYTHON_VERSION_MAJOR and
PYTHON_VERSION_MINOR to prefer these versions when looking for the
header path and library.  The Python_ADDITIONAL_VERSIONS variable has
priority over the interpreter version.

Co-Author: Adam Wolf
Co-Author: Gert Wollny <[email protected]>

Modules/FindPythonLibs.cmake

@@ -80,10 +80,15 @@ endif()
 
 # Set up the versions we know about, in the order we will search. Always add
 # the user supplied additional versions to the front.
-set(_Python_VERSIONS
-  ${Python_ADDITIONAL_VERSIONS}
-  ${_PYTHON_FIND_OTHER_VERSIONS}
-  )
+# If FindPythonInterp has already found the major and minor version,
+# insert that version between the user supplied versions and the stock
+# version list.
+find_package(PythonInterp QUIET)
+set(_Python_VERSIONS ${Python_ADDITIONAL_VERSIONS})
+if(DEFINED PYTHON_VERSION_MAJOR AND DEFINED PYTHON_VERSION_MINOR)
+  list(APPEND _Python_VERSIONS ${PYTHON_VERSION_MAJOR}.${PYTHON_VERSION_MINOR})
+endif()
+list(APPEND _Python_VERSIONS ${_PYTHON_FIND_OTHER_VERSIONS})
 
 unset(_PYTHON_FIND_OTHER_VERSIONS)
 unset(_PYTHON1_VERSIONS)