Take into account GSL installations not in standard path

brian2/codegen/runtime/GSLcython_rt/GSLcython_rt.py

@@ -3,6 +3,8 @@
 GNU Scientific Library (GSL)
 """
 
+import os
+import subprocess
 import sys
 from distutils.errors import CompileError
 
@@ -47,6 +49,15 @@ def compile(self):
             self.define_macros += [("WIN32", "1"), ("GSL_DLL", "1")]
         if prefs.GSL.directory is not None:
             self.include_dirs += [prefs.GSL.directory]
+        else:
+            try:
+                p = subprocess.run(
+                    "gsl-config --prefix", shell=True, capture_output=True, check=True
+                )
+                output = p.stdout.decode("utf-8").strip()
+                self.include_dirs += [os.path.join(output, "include")]
+            except subprocess.CalledProcessError:
+                pass  # ignore
         try:
             super().compile()
         except CompileError as err:

brian2/stateupdaters/GSL.py

@@ -2,7 +2,8 @@
 Module containg the StateUpdateMethod for integration using the ODE solver
 provided in the GNU Scientific Library (GSL)
 """
-
+import os
+import subprocess
 import sys
 
 from brian2.utils.logger import get_logger
@@ -105,6 +106,17 @@ def get_codeobj_class(self):
                     device.define_macros += [("WIN32", "1"), ("GSL_DLL", "1")]
                 if prefs.GSL.directory is not None:
                     device.include_dirs += [prefs.GSL.directory]
+                try:
+                    p = subprocess.run(
+                        "gsl-config --prefix",
+                        shell=True,
+                        capture_output=True,
+                        check=True,
+                    )
+                    output = p.stdout.decode("utf-8").strip()
+                    device.include_dirs += [os.path.join(output, "include")]
+                except subprocess.CalledProcessError:
+                    pass  # ignore
             return GSLCPPStandaloneCodeObject
 
         elif isinstance(device, RuntimeDevice):