Make Nim/SV interface work even when Nim is compiled to C++

Thanks to @jrfondren.

Ref:
- https://gitter.im/nim-lang/Nim?at=5cc377b03d78aa6c039e4bb7
- nim-lang/Nim#10578

hello_from_nim/Makefile

@@ -1,8 +1,14 @@
-# Time-stamp: <2019-01-22 13:40:01 kmodi>
+# Time-stamp: <2019-04-26 17:48:17 kmodi>
 
 .DEFAULT_GOAL := default
 
 GIT_ROOT = $(shell git rev-parse --show-toplevel)
 include $(GIT_ROOT)/makefile
 
+# Compiles to C
 default: libdpi nc
+
+# Compiles to C++
+cpp:
+	$(MAKE) libdpi NIM_COMPILES_TO=cpp
+	$(MAKE) nc

hello_from_nim/libdpi.nim

@@ -1,2 +1,8 @@
+when defined(cpp):
+  {.push codegenDecl: """extern "C" $1 $2 $3""".}
+
 proc hello() {.exportc.} =
   echo "Hello from Nim!"
+
+when defined(cpp):
+  {.pop.}

makefile

@@ -1,4 +1,4 @@
-# Time-stamp: <2019-04-26 15:05:09 kmodi>
+# Time-stamp: <2019-04-26 17:46:54 kmodi>
 # Author    : Kaushal Modi
 
 FILES   = tb.sv
@@ -23,6 +23,8 @@ else
 	GCC_ARCH_FLAG := -m32
 endif
 
+# Possible values of NIM_COMPILES_TO: c, cpp
+NIM_COMPILES_TO ?= c
 NIM_GC ?=
 NIM_DEFINES =
 
@@ -39,7 +41,7 @@ clean:
 # https://irclogs.nim-lang.org/21-01-2019.html#17:16:39
 libdpi:
 	@find . \( -name libdpi.o -o -name libdpi.so \) -delete
-	nim c --out:libdpi.so --app:lib --nimcache:./.nimcache $(NIM_ARCH_FLAGS) $(NIM_GC) --hint[Processing]:off $(NIM_DEFINES) libdpi.nim
+	nim $(NIM_COMPILES_TO) --out:libdpi.so --app:lib --nimcache:./.nimcache $(NIM_ARCH_FLAGS) $(NIM_GC) --hint[Processing]:off $(NIM_DEFINES) libdpi.nim
 
 nc:
 	xrun -sv $(NC_ARCH_FLAGS) \