Makefile: fix check-libroach with linux release toolchain

The Linux release toolchain uses an ancient version of glibc that
requires linking against -lrt. Teach CMake to link test binaries
accordingly on Linux.

The Linux release toolchain also uses a very recent version of libstdc++
that must be statically linked. Plumb the link flags down into CMake via
the LDFLAGS variable. (Previously they were only passed down to links
invoked by Go via the -extldflags flag in the LINKFLAGS variable.)

Release note: None

Makefile

@@ -148,19 +148,22 @@ else ifeq ($(TYPE),release-linux-gnu)
 # this toolchain's libstdc++ version is quite recent and must be statically
 # linked to avoid depending on the target's available libstdc++.
 XHOST_TRIPLE := x86_64-unknown-linux-gnu
-override LINKFLAGS += -extldflags "-static-libgcc -static-libstdc++"
+override LDFLAGS = -static-libgcc -static-libstdc++
+override LINKFLAGS += -extldflags "$(LDFLAGS)"
 override SUFFIX := $(SUFFIX)-linux-2.6.32-gnu-amd64
 BUILD_TYPE := release
 else ifeq ($(TYPE),release-linux-musl)
 XHOST_TRIPLE := x86_64-unknown-linux-musl
-override LINKFLAGS += -extldflags "-static"
+override LDFLAGS = -static
+override LINKFLAGS += -extldflags "$(LDFLAGS)"
 override SUFFIX := $(SUFFIX)-linux-2.6.32-musl-amd64
 BUILD_TYPE := release
 else ifeq ($(TYPE),release-aarch64-linux)
 XGOARCH := arm64
 export CGO_ENABLED := 1
 XHOST_TRIPLE := aarch64-unknown-linux-gnueabi
-override LINKFLAGS += -extldflags "-static-libgcc -static-libstdc++"
+override LDFLAGS = -static-libgcc -static-libstdc++
+override LINKFLAGS += -extldflags "$(LDFLAGS)"
 override SUFFIX := $(SUFFIX)-linux-3.7.10-gnu-aarch64
 BUILD_TYPE := release
 else ifeq ($(TYPE),release-darwin)
@@ -174,7 +177,8 @@ XGOOS := windows
 export CGO_ENABLED := 1
 XHOST_TRIPLE := x86_64-w64-mingw32
 override SUFFIX := $(SUFFIX)-windows-6.2-amd64
-override LINKFLAGS += -extldflags "-static"
+override LDFLAGS += -static
+override LINKFLAGS += -extldflags "$(LDFLAGS)"
 BUILD_TYPE := release
 else
 $(error unknown build type $(TYPE))

c-deps/libroach/CMakeLists.txt

@@ -152,6 +152,10 @@ foreach(tsrc ${tests})
     ${SNAPPY_LIB}
   )
 
+  if(${CMAKE_SYSTEM_NAME} MATCHES "Linux")
+    target_link_libraries(${tname} rt)
+  endif()
+
   set_target_properties(${tname} PROPERTIES
     CXX_STANDARD 11
     CXX_STANDARD_REQUIRED YES