Allow users to pass empty AMDGCN list

This allows users to handle cases like LLVM, where building with GPU
support is optional, but users might still want to install the software
without GPU support.

Signed-off-by: Jan Andre Reuter <jan@zyten.de>

Author: Thyre

easybuild/tools/options.py

@@ -999,6 +999,9 @@ def validate(self):
         # Support accelerators using the gfx[...] naming scheme.
         # This applies to all AMD GPUs since Southern Islands (2013)
         # For more information: https://llvm.org/docs/AMDGPUUsage.html#processors
+        # Allow users to pass --amdgcn-capabilities=, which will be mapped to not passing any
+        if self.options.amdgcn_capabilities == ['']:
+            self.options.amdgcn_capabilities = None
         if self.options.amdgcn_capabilities:
             # General accelerator naming convention
             amdgcn_cc_regex = re.compile(r'gfx[0-9]+[a-z]?$')