implements https://github.com/nim-lang/RFCs/issues/260 (#15505)

* implements https://github.com/nim-lang/RFCs/issues/260

* added a test case

tests/pragmas/cfunction.c

@@ -0,0 +1,4 @@
+
+int cfunction(void) {
+  return NUMBER_HERE;
+}

tests/pragmas/tcompile_pragma.nim

@@ -0,0 +1,9 @@
+discard """
+  output: '''34'''
+"""
+
+{.compile("cfunction.c", "-DNUMBER_HERE=34").}
+
+proc cfunction(): cint {.importc.}
+
+echo cfunction()