Fix cosmopolitan.h for C++

We now have an integration test that runs the amalgamated sources
through a C++ compiler, to prevent the recurrence of such issues.

Fixes #38
Thanks @gbbnfhb for the report!

libc/nt/systeminfo.h

@@ -17,5 +17,6 @@ bool32 GetComputerNameEx(/* enum/computernameformat.h */ int NameType,
 #if ShouldUseMsabiAttribute()
 #include "libc/nt/thunk/systeminfo.inc"
 #endif /* ShouldUseMsabiAttribute() */
+COSMOPOLITAN_C_END_
 #endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
 #endif /* COSMOPOLITAN_LIBC_NT_INFO_H_ */

test/libc/release/smokecxx.cc

@@ -0,0 +1,11 @@
+int main() {
+  int rc;
+  FILE *f;
+  f = fopen("/dev/null", "w");
+  fprintf(f, "hello world\n");
+  fclose(f);
+  rc = system("exit 42");
+  CHECK_NE(-1, rc);
+  CHECK_EQ(42, WEXITSTATUS(rc));
+  return 0;
+}

test/libc/release/test.mk

@@ -38,7 +38,38 @@ o/$(MODE)/test/libc/release/smoke.com.dbg:			\
 		o/$(MODE)/ape/ape.o				\
 		o/$(MODE)/cosmopolitan.a
 
+o/$(MODE)/test/libc/release/smokecxx.com:			\
+		o/$(MODE)/test/libc/release/smokecxx.com.dbg
+	@objcopy -SO binary $< $@
+
+o/$(MODE)/test/libc/release/smokecxx.com.dbg:			\
+		test/libc/release/smokecxx.cc			\
+		o/cosmopolitan.h				\
+		o/$(MODE)/ape/ape.lds				\
+		o/$(MODE)/libc/crt/crt.o			\
+		o/$(MODE)/ape/ape.o				\
+		o/$(MODE)/cosmopolitan.a
+	@ACTION=CXX build/compile $(CXX)			\
+		-o $@						\
+		-Os						\
+		-static						\
+		-no-pie						\
+		-fno-pie					\
+		-nostdlib					\
+		-nostdinc					\
+		-mno-red-zone					\
+		-Wl,--gc-sections				\
+		-Wl,-z,max-page-size=0x1000			\
+		-Wl,-T,o/$(MODE)/ape/ape.lds			\
+		-include o/cosmopolitan.h			\
+		test/libc/release/smokecxx.cc			\
+		o/$(MODE)/libc/crt/crt.o			\
+		o/$(MODE)/ape/ape.o				\
+		o/$(MODE)/cosmopolitan.a
+
 .PHONY: o/$(MODE)/test/libc/release
 o/$(MODE)/test/libc/release:					\
 		o/$(MODE)/test/libc/release/smoke.com		\
-		o/$(MODE)/test/libc/release/smoke.com.runs
+		o/$(MODE)/test/libc/release/smoke.com.runs	\
+		o/$(MODE)/test/libc/release/smokecxx.com	\
+		o/$(MODE)/test/libc/release/smokecxx.com.runs