Fix issue #3795 - Fix LDC linked dynamically

with Phobos and DRuntime when built with GDMD.

Prepend arguments starting with "-B" in D_LINKER_ARGS with "-Wl," when
using GDMD. The latter is needed because the arguments reported by GDMD
are what is passed to COLLECT2 while D_LINKER_ARGS are later passed to
CXX. The problem with that is that without "-Wl," prefix the -Bstatic
and -Bdynamic arguments before and after -lgphobos and -libgdruntime are
dropped. And that is a problem because without these the produced LDC is
linked dynamically to libgphobos and libgdruntime so these have to be
available whereever LDC is used.

Once libgphobos and libgdruntime are linked statically symbol conflicts
for _d_allocmemory, _d_newclass, _d_newitemiT and _d_newitemT symbols
were revealed which are fixed by checking if these symbols are marked as
weak in libgdruntime.a and if not "-Wl,-allow-multiple-definition" link
option is added to avoid link failure.

Author: Dentcho Bankov

cmake/Modules/ExtractDMDSystemLinker.cmake

@@ -66,12 +66,47 @@ list(REMOVE_AT linker_line 0)
 
 if("${D_COMPILER_ID}" STREQUAL "GDMD")
     # Filter linker arguments for those we know can be safely reused
+    set(STATIC OFF)
     set(D_LINKER_ARGS)
     foreach(arg ${linker_line})
-        if("${arg}" MATCHES ^-L.*|^-l.*|^-B.*)
+        if("${arg}" MATCHES ^-L.*|^-l.*)
             list(APPEND D_LINKER_ARGS "${arg}")
+            if(STATIC)
+                if("${arg}" STREQUAL -lgphobos)
+                    set(STATIC_GPHOBOS ON)
+                endif()
+            endif()
+        elseif("${arg}" MATCHES ^-B.*)
+            list(APPEND D_LINKER_ARGS "-Wl,${arg}")
+            if("${arg}" STREQUAL -Bstatic)
+                set(STATIC ON)
+            elseif("${arg}" STREQUAL -Bdynamic)
+                set(STATIC OFF)
+            endif()
         endif()
     endforeach()
+    if(STATIC_GPHOBOS)
+        execute_process(
+            COMMAND "${D_COMPILER}" -q,-print-file-name=libgdruntime.a
+            COMMAND "head" -n1
+            COMMAND "xargs" nm -gC
+            COMMAND "grep"
+                -e _d_allocmemory
+                -e _d_newclass
+                -e _d_newitemiT
+                -e _d_newitemT
+            COMMAND "cut" -d " " -f 2
+            COMMAND "sort"
+            COMMAND "uniq"
+            OUTPUT_VARIABLE LIB_GDRUNTIME_LIFETIME_SYMBOL_TYPES
+            ERROR_QUIET)
+        string(STRIP
+            ${LIB_GDRUNTIME_LIFETIME_SYMBOL_TYPES}
+            LIB_GDRUNTIME_LIFETIME_SYMBOL_TYPES)
+        if(NOT LIB_GDRUNTIME_LIFETIME_SYMBOL_TYPES STREQUAL W)
+            list(APPEND D_LINKER_ARGS "-Wl,--allow-multiple-definition")
+        endif()
+    endif()
 else()
     set(D_LINKER_ARGS ${linker_line})
 endif()