Fix documentation build (#71)

doc/CMakeLists.txt

@@ -21,11 +21,11 @@ foreach (f ${files})
    endif ()
 endforeach ()
 
-# Add a command to extract the amplgsl documentation from amplgsl.c.
+# Add a command to extract the amplgsl documentation from amplgsl.cc.
 add_custom_command(OUTPUT ${amplgsl_docs}
-  COMMAND ${BUILD_DOCS} extract-docs ${amplgsl_c}
+  COMMAND ${BUILD_DOCS} extract-docs ${amplgsl_src}
   WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
-  DEPENDS ${BUILD_DOCS} ${amplgsl_c})
+  DEPENDS ${BUILD_DOCS} ${amplgsl_src})
 
 # Add commands to copy .rst files to the binary directory.
 # This is needed because some of the .rst files are autogenerated and