Use LIBPL variable to find Python library

Search Python library in two directories on Unix:
 - the one returned by the LIBPL variable (for Python system);
 - the parent directory of the standard Python library modules (for
   pyenv).
Do not search Python library through subdirectories as it can lead
to find the wrong Python library (the debug version for instance).

build.py

@@ -150,12 +150,19 @@ def CheckOutput( *popen_args, **kwargs ):
   return output
 
 
+def GetPossiblePythonLibraryDirectories():
+  library_dir = p.dirname( sysconfig.get_python_lib( standard_lib = True ) )
+  if OnWindows():
+    return [ p.join( library_dir, 'libs' ) ]
+  # On pyenv, there is no Python dynamic library in the directory returned by
+  # the LIBPL variable. Such library is located in the parent folder of the
+  # standard Python library modules.
+  return [ sysconfig.get_config_var( 'LIBPL' ), library_dir ]
+
+
 def FindPythonLibraries():
   include_dir = sysconfig.get_python_inc()
-  # get_python_lib with the standard_lib parameter set to True returns the
-  # standard Python modules directory. Python libraries should always be in
-  # the parent directory or one of its subdirectories.
-  library_dir = p.dirname( sysconfig.get_python_lib( standard_lib = True ) )
+  library_dirs = GetPossiblePythonLibraryDirectories()
 
   # Since ycmd is compiled as a dynamic library, we can't link it to a Python
   # static library. If we try, the following error will occur on Mac:
@@ -183,17 +190,15 @@ def FindPythonLibraries():
     major = PY_MAJOR, minor = PY_MINOR ), re.X )
   static_libraries = []
 
-  # We search the Python libraries through the library directory and its
-  # subdirectories.
-  for root, dirs, files in os.walk( library_dir ):
+  for library_dir in library_dirs:
     # Files are sorted so that we found the non-versioned Python library before
     # the versioned one.
-    for filename in sorted( files ):
+    for filename in sorted( os.listdir( library_dir ) ):
       if dynamic_name.match( filename ):
-        return p.join( root, filename ), include_dir
+        return p.join( library_dir, filename ), include_dir
 
       if static_name.match( filename ):
-        static_libraries.append( p.join( root, filename ) )
+        static_libraries.append( p.join( library_dir, filename ) )
 
   if static_libraries and not OnWindows():
     dynamic_flag = ( '--enable-framework' if OnMac() else