* Verify that /proc is mounted. It makes some unexpected issues when

building from a chroot without /proc

debian/changelog

@@ -8,6 +8,8 @@ llvm-toolchain-snapshot (1:13~++20210218085556+fcdef15d77bd-1~exp1) UNRELEASED;
   * Revert https://reviews.llvm.org/D95727 as it is causing some issues like:
     - /usr/bin/ld: ./build-llvm/lib/ExecutionEngine/Interpreter/./llvm/lib/ExecutionEngine/Interpreter/ExternalFunctions.cpp:135: undefined reference to `ffi_type_pointer'
     -  Host compiler must support std::atomic!
+  * Verify that /proc is mounted. It makes some unexpected issues when
+    building from a chroot without /proc
 
   [ John Paul Adrian Glaubitz ]
   * Add upstream patch D98574 to fix clang macro definitions on sparc64

debian/rules

@@ -311,6 +311,12 @@ endif
 PROJECTS_LIST := $(shell echo "$(PROJECTS)"|sed -e "s| ||g")
 
 preconfigure:
+
+	if ! grep -qs '/proc ' /proc/mounts; then \
+		echo "/proc needs to be mounted"; \
+		exit 1; \
+	fi
+
 	@echo "DEB_HOST_MULTIARCH=$(DEB_HOST_MULTIARCH)"
 	@echo "DEB_HOST_GNU_TYPE=$(DEB_HOST_GNU_TYPE)"
 	@echo "DEB_HOST_ARCH_BITS=$(DEB_HOST_ARCH_BITS)"