CyOpenGL compilation: compile CyOpenGL even if there is no Tk as we c…

…an still compile it, even though we cannot import it later. Raise an exception if CyOpenGL headers cannot be found if SNAPPY_ALWAYS_BUILD_CYOPENGL is set.

setup.py

@@ -501,6 +501,22 @@ def add(self, source_file, dependency_mod_time=0.0):
     language='c++'
 )
 
+CyOpenGL_has_headers = False
+if sys.platform == 'win32':
+    # We use glew, so we do not need GL headers from somewhere
+    # else
+    CyOpenGL_has_headers = True
+else:
+    header = 'gl.h'
+    CyOpenGL_has_headers = any(
+        exists(os.path.join(path, header))
+        for path in CyOpenGL_includes)
+    if not CyOpenGL_has_headers:
+        print("***WARNING***: Not building CyOpenGL so many graphics "
+              "features will not be available.")
+        print("This is because the OpenGL header %s was not found." % header)
+        print("The following directories were searched: %s." % (
+            ', '.join(CyOpenGL_includes)))
 
 # Twister
 
@@ -536,29 +552,12 @@ def add(self, source_file, dependency_mod_time=0.0):
     else:
         install_requires.append('ipython>=1.0')
 
-# Determine whether we will be able to activate the GUI code
-
-try:
-    import tkinter as Tk
-except ImportError:
-    Tk = None
-
-if sys.platform == 'win32':
+if CyOpenGL_has_headers:
     ext_modules.append(CyOpenGL)
-elif Tk is not None:
-    missing = {}
-    for header in ['gl.h']:
-        results = [exists(os.path.join(path, header))
-                   for path in CyOpenGL_includes]
-        missing[header] = (True in results)
-    if False in missing.values():
-        print("***WARNING***: OpenGL headers not found, "
-              "not building CyOpenGL, "
-              "will disable some graphics features.")
-    else:
-        ext_modules.append(CyOpenGL)
-else:
-    print("***WARNING**: Tkinter not installed, GUI won't work")
+elif (os.environ.get('SNAPPY_ALWAYS_BUILD_CYOPENGL', 'False')
+      not in ['0', 'false', 'False']):
+    raise RuntimeError('Could not find CyOpenGL requirements but '
+                       'SNAPPY_ALWAYS_BUILD_CYOPENGL is set.')
 
 # Get version number:
 exec(open('python/version.py').read())