Fix macOS 26 CLING runtime dictionary errors caused by Homebrew/SDK header conflicts

macOS 26 (Darwin 25.5.0) introduced stricter Clang module map rules in
DarwinFoundation1.modulemap: the '_c_standard_library_obsolete' module
now requires the 'found_incompatible_headers__check_search_paths' feature,
which CLING's runtime interpreter does not set. When fftw3.h (from
/opt/homebrew/include) was parsed by CLING via the inlined PStartupHandler
dictionary payload, it triggered this module map error, causing all
PStartupHandler signal/slot connections to fail.

Two fixes:
- PMusr.h: guard '#include "fftw3.h"' with '#ifndef __CLING__'. No fftw
  types appear in PMusr.h class definitions, so CLING does not need it
  for reflection. This is the primary runtime fix.
- CMakeLists.txt (src/classes and all src/external libs): replace
  '-I${FFTW3_INCLUDE}', '-I${Boost_INCLUDE_DIRS}', '-I${GSL_INCLUDE_DIRS}',
  and '-I${ROOT_INCLUDE_DIRS}' with '-isystem' in all root_generate_dictionary
  OPTIONS blocks, and change 'include_directories(${FFTW3_INCLUDE})' to
  'include_directories(SYSTEM ...)'. Internal project paths (MUSRFIT_INC,
  BMW_TOOLS_INC, NONLOCAL_INC, CMAKE_CURRENT_SOURCE_DIR, etc.) keep '-I'.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/classes/CMakeLists.txt

@@ -6,14 +6,14 @@ set(MUSRFIT_INC ${CMAKE_SOURCE_DIR}/src/include)
 # Hence, target_include_directories cannot be used here because, targets are 
 # setup only afterwards.
 include_directories(${MUSRFIT_INC})
-include_directories(${FFTW3_INC})
+include_directories(SYSTEM ${FFTW3_INCLUDE})
 
 root_generate_dictionary(
   PFourierCanvasDict 
   PFourierCanvas.h 
   OPTIONS 
     -I${MUSRFIT_INC} 
-    -I${FFTW3_INCLUDE}
+    -isystem ${FFTW3_INCLUDE}
     -inlineInputHeader
   LINKDEF ${MUSRFIT_INC}/PFourierCanvasLinkDef.h
   MODULE PFourierCanvas
@@ -23,7 +23,7 @@ root_generate_dictionary(
   PMsgBox.h  
   LINKDEF ${MUSRFIT_INC}/PMsgBoxLinkDef.h
   OPTIONS 
-    -I${Boost_INCLUDE_DIRS} -I${FFTW3_INCLUDE} -I${MUSRFIT_INC}
+    -isystem ${Boost_INCLUDE_DIRS} -isystem ${FFTW3_INCLUDE} -I${MUSRFIT_INC}
     -inlineInputHeader
   MODULE PMsgBox
 )
@@ -32,7 +32,7 @@ root_generate_dictionary(
   PMusrCanvas.h  
   LINKDEF ${MUSRFIT_INC}/PMusrCanvasLinkDef.h
   OPTIONS 
-    -I${Boost_INCLUDE_DIRS} -I${FFTW3_INCLUDE} -I${MUSRFIT_INC}
+    -isystem ${Boost_INCLUDE_DIRS} -isystem ${FFTW3_INCLUDE} -I${MUSRFIT_INC}
     -inlineInputHeader
   MODULE PMusrCanvas
 )
@@ -41,7 +41,7 @@ root_generate_dictionary(
   PMusrT0.h
   LINKDEF ${MUSRFIT_INC}/PMusrT0LinkDef.h
   OPTIONS 
-    -I${Boost_INCLUDE_DIRS} -I${FFTW3_INCLUDE} -I${MUSRFIT_INC}
+    -isystem ${Boost_INCLUDE_DIRS} -isystem ${FFTW3_INCLUDE} -I${MUSRFIT_INC}
     -inlineInputHeader
   MODULE PMusrT0
 )
@@ -50,7 +50,7 @@ root_generate_dictionary(
   PStartupHandler.h 
   LINKDEF ${MUSRFIT_INC}/PStartupHandlerLinkDef.h
   OPTIONS 
-    -I${FFTW3_INCLUDE} -I${MUSRFIT_INC}
+    -isystem ${FFTW3_INCLUDE} -I${MUSRFIT_INC}
     -inlineInputHeader
   MODULE PStartupHandler
 )
@@ -60,7 +60,7 @@ root_generate_dictionary(
   LINKDEF ${MUSRFIT_INC}/PUserFcnBaseLinkDef.h
   OPTIONS
     -inlineInputHeader
-    -I${FFTW3_INCLUDE} -I${MUSRFIT_INC}
+    -isystem ${FFTW3_INCLUDE} -I${MUSRFIT_INC}
   MODULE PUserFcnBase
 )
 root_generate_dictionary(
@@ -69,7 +69,7 @@ root_generate_dictionary(
   OPTIONS 
     -I${NONLOCAL_INC} 
     -I${MUSRFIT_INC}
-    -I${FFTW3_INCLUDE}
+    -isystem ${FFTW3_INCLUDE}
     -I${CMAKE_CURRENT_SOURCE_DIR}
     -inlineInputHeader
   LINKDEF ${MUSRFIT_INC}/PRgeHandlerLinkDef.h
@@ -129,7 +129,7 @@ add_library(PMusr SHARED
 )
 #--- make sure that the include directory is found ----------------------------
 target_include_directories(
-  PMusr BEFORE PRIVATE
+  PMusr SYSTEM BEFORE PRIVATE
     $<BUILD_INTERFACE:${DKS_INCLUDE_DIR}> 
     $<BUILD_INTERFACE:${Boost_INCLUDE_DIRS}>
     $<BUILD_INTERFACE:${FFTW3_INCLUDE}>