Always enable the dotnet module. (#222)

When building prior to this commit the dotnet module would be disabled unless
you explicitly asked for it. This is the opposite of what the default is in
libyara. Fix it by always building the dotnet module.

Tested with:

```
wxs@mbp yara-python % PYTHONPATH=build/lib.macosx-10.9-universal2-3.9 python3 -c 'import yara; print(yara.modules)'
['tests', 'pe', 'elf', 'math', 'time', 'console', 'string', 'dotnet', 'hash']
wxs@mbp yara-python %
```

Without this change the dotnet module would not be in the list.

setup.py

@@ -34,7 +34,6 @@ OPTIONS = [
    ('dynamic-linking', None, 'link dynamically against libyara'),
    ('enable-cuckoo', None, 'enable "cuckoo" module'),
    ('enable-magic', None, 'enable "magic" module'),
-   ('enable-dotnet', None, 'enable "dotnet" module'),
    ('enable-dex', None, 'enable "dex" module'),
    ('enable-macho', None, 'enable "macho" module'),
    ('enable-profiling', None, 'enable profiling features')]
@@ -44,7 +43,6 @@ BOOLEAN_OPTIONS = [
     'dynamic-linking',
     'enable-cuckoo',
     'enable-magic',
-    'enable-dotnet',
     'enable-dex',
     'enable-macho',
     'enable-profiling']
@@ -119,7 +117,6 @@ class BuildCommand(build):
     self.dynamic_linking = None
     self.enable_magic = None
     self.enable_cuckoo = None
-    self.enable_dotnet = None
     self.enable_dex = None
     self.enable_macho = None
     self.enable_profiling = None
@@ -141,7 +138,6 @@ class BuildExtCommand(build_ext):
     self.dynamic_linking = None
     self.enable_magic = None
     self.enable_cuckoo = None
-    self.enable_dotnet = None
     self.enable_dex = None
     self.enable_macho = None
     self.enable_profiling = None
@@ -157,7 +153,6 @@ class BuildExtCommand(build_ext):
         ('dynamic_linking', 'dynamic_linking'),
         ('enable_magic', 'enable_magic'),
         ('enable_cuckoo', 'enable_cuckoo'),
-        ('enable_dotnet', 'enable_dotnet'),
         ('enable_dex', 'enable_dex'),
         ('enable_macho', 'enable_macho'),
         ('enable_profiling', 'enable_profiling'))
@@ -168,9 +163,6 @@ class BuildExtCommand(build_ext):
     if self.enable_cuckoo and self.dynamic_linking:
       raise distutils.errors.DistutilsOptionError(
           '--enable-cuckoo can''t be used with --dynamic-linking')
-    if self.enable_dotnet and self.dynamic_linking:
-      raise distutils.errors.DistutilsOptionError(
-          '--enable-dotnet can''t be used with --dynamic-linking')
     if self.enable_dex and self.dynamic_linking:
       raise distutils.errors.DistutilsOptionError(
           '--enable-dex can''t be used with --dynamic-linking')
@@ -314,11 +306,6 @@ class BuildExtCommand(build_ext):
       else:
         exclusions.append('yara/libyara/modules/cuckoo/cuckoo.c')
 
-      if self.enable_dotnet:
-        module.define_macros.append(('DOTNET_MODULE', '1'))
-      else:
-        exclusions.append('yara/libyara/modules/dotnet/dotnet.c')
-
       if self.enable_dex:
         module.define_macros.append(('DEX_MODULE', '1'))
       else:
@@ -333,6 +320,9 @@ class BuildExtCommand(build_ext):
       exclusions.append('yara/libyara/modules/pb_tests/pb_tests.c')
       exclusions.append('yara/libyara/modules/pb_tests/pb_tests.pb-c.c')
 
+      # Always turn on the DOTNET module.
+      module.define_macros.append(('DOTNET_MODULE', '1'))
+
       exclusions = [os.path.normpath(x) for x in exclusions]
 
       for directory, _, files in os.walk('yara/libyara/'):