BuildPDAF: Add -fcommon to Fortran/C optimization flags (#79)

Goal: remove multiple-definition errors for GCC compilers.

Author: jjokella

cmake/BuildPDAF.cmake

@@ -145,6 +145,7 @@ elseif (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
   list(APPEND PDAF_FOPT "-ffree-form")
   # list(APPEND PDAF_FOPT "-fopenmp")
   list(APPEND PDAF_FOPT "-fallow-argument-mismatch")
+  list(APPEND PDAF_FOPT "-fcommon")
 
 else()
   message(FATAL_ERROR "Unsupported CMAKE_CXX_COMPILER_ID: ${CMAKE_CXX_COMPILER_ID}")
@@ -192,6 +193,7 @@ elseif (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
   list(APPEND PDAF_COPT "-fno-automatic")
   list(APPEND PDAF_COPT "-finit-local-zero")
   list(APPEND PDAF_COPT "-mcmodel=large")
+  list(APPEND PDAF_COPT "-fcommon")
 
 else()
   message(FATAL_ERROR "Unsupported CMAKE_CXX_COMPILER_ID: ${CMAKE_CXX_COMPILER_ID}")