diff --git a/Makefile b/Makefile
index 3df0ab5c76..07902eb592 100644
--- a/Makefile
+++ b/Makefile
@@ -1218,7 +1218,7 @@ ARFLAGS = rcs
 PTHREAD_CFLAGS =
 
 # For the 'sparse' target
-SPARSE_FLAGS ?=
+SPARSE_FLAGS ?= -std=gnu99
 SP_EXTRA_FLAGS = -Wno-universal-initializer
 
 # For the 'coccicheck' target; setting SPATCH_BATCH_SIZE higher will
diff --git a/contrib/buildsystems/CMakeLists.txt b/contrib/buildsystems/CMakeLists.txt
index 171b4124af..0945a83223 100644
--- a/contrib/buildsystems/CMakeLists.txt
+++ b/contrib/buildsystems/CMakeLists.txt
@@ -208,7 +208,7 @@ endif()
 if(CMAKE_C_COMPILER_ID STREQUAL "MSVC")
 	set(CMAKE_RUNTIME_OUTPUT_DIRECTORY_DEBUG ${CMAKE_BINARY_DIR})
 	set(CMAKE_RUNTIME_OUTPUT_DIRECTORY_RELEASE ${CMAKE_BINARY_DIR})
-	add_compile_options(/MP)
+	add_compile_options(/MP /std:c11)
 endif()
 
 #default behaviour
diff --git a/git-compat-util.h b/git-compat-util.h
index b46605300a..19943e214b 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -1,6 +1,19 @@
 #ifndef GIT_COMPAT_UTIL_H
 #define GIT_COMPAT_UTIL_H
 
+#if __STDC_VERSION__ - 0 < 199901L
+/*
+ * Git is in a testing period for mandatory C99 support in the compiler.  If
+ * your compiler is reasonably recent, you can try to enable C99 support (or,
+ * for MSVC, C11 support).  If you encounter a problem and can't enable C99
+ * support with your compiler (such as with "-std=gnu99") and don't have access
+ * to one with this support, such as GCC or Clang, you can remove this #if
+ * directive, but please report the details of your system to
+ * git@vger.kernel.org.
+ */
+#error "Required C99 support is in a test phase.  Please see git-compat-util.h for more details."
+#endif
+
 #ifdef USE_MSVC_CRTDBG
 /*
  * For these to work they must appear very early in each