MainBuildRules: Fail if the generated struct-offsets header is empty.

In some LTO-related experiments, it came out empty due to GCC
not generating actual assembly but rather intermediate code,
resulting in various later targets failing with quite literally
thousands of errors.

build/jam/MainBuildRules

@@ -383,6 +383,7 @@ actions CreateAsmStructOffsetsHeader1
  	$(C++) -S "$(2)" $(C++FLAGS) $(CCDEFS) $(CCHDRS) -o - \
 		| $(SED) 's/@define/#define/g' | grep "#define" \
 		| $(SED) -e 's/[\$\#]\([0-9]\)/\1/' > "$(1)"
+	grep -q "#define" "$(1)"
 }
 
 rule MergeObjectFromObjects
@@ -420,7 +421,7 @@ rule MergeObjectFromObjects
 
 actions MergeObjectFromObjects1
 {
-	$(LINK) $(LINKFLAGS) -r $(2) -o $(1) ;
+	$(LINK) $(LINKFLAGS) -r $(2) -o $(1)
 }
 
 rule MergeObject