build: rename --enable-shared switch

Rename it to --enable-libmpv-shared. The option name didn't really
tell much. When we add the possibility to create a static library,
it would also be bad if that were named --enable-static (because it
would sound like it does what --static-build does).

wscript

@@ -10,14 +10,14 @@ from waftools.checks.custom import *
 
 build_options = [
     {
-        'name': '--shared',
+        'name': '--libmpv-shared',
         'desc': 'shared library',
         'default': 'disable',
         'func': check_true
     }, {
         'name': '--client-api-examples',
         'desc': 'build client API examples',
-        'deps': ['shared'],
+        'deps': ['libmpv-shared'],
         'func': check_true
     }, {
         'name': '--static-build',

wscript_build.py

@@ -438,7 +438,7 @@ def build(ctx):
         **cprog_kwargs
     )
 
-    if ctx.dependency_satisfied('shared'):
+    if ctx.dependency_satisfied('libmpv-shared'):
         ctx.load("syms")
         ctx(
             target       = "mpv",