Set CFLAGS/LDFLAGS/etc according to Debian policy

Debian wants to build all binaries with particular hardening flags.
The Rust makefiles are inconsistent in which architectures they
correctly include CFLAGS/etc from the enivoronment (see mk/cfg/*).

This patch adds LDFLAGS, and then unconditionally prepends
CFLAGS/LDFLAGS/etc to the build commands.

configure

@@ -1053,12 +1053,13 @@ fi
 # point in the script; after this point, script logic should inspect
 # $CFG_USING_CLANG rather than $CFG_ENABLE_CLANG.
 
-# Set CFG_{CC,CXX,CPP,CFLAGS,CXXFLAGS}
+# Set CFG_{CC,CXX,CPP,CFLAGS,CXXFLAGS,LDFLAGS}
 envopt CC
 envopt CXX
 envopt CPP
 envopt CFLAGS
 envopt CXXFLAGS
+envopt LDFLAGS
 
 # stdc++ name in use
 # used to manage non-standard name (on OpenBSD for example)
@@ -1667,9 +1668,9 @@ do
 
         CXX=$LLVM_CXX
         CC=$LLVM_CC
-        CFLAGS=$LLVM_CFLAGS
-        CXXFLAGS=$LLVM_CXXFLAGS
-        LDFLAGS=$LLVM_LDFLAGS
+        CFLAGS="$CFLAGS $LLVM_CFLAGS"
+        CXXFLAGS="$CXXFLAGS $LLVM_CXXFLAGS"
+        LDFLAGS="$LDFLAGS $LLVM_LDFLAGS"
 
         if [ -z "$CFG_DISABLE_LIBCPP" ] && [ -n "$CFG_USING_CLANG" ]; then
             LLVM_OPTS="$LLVM_OPTS --enable-libcpp"

mk/platform.mk

@@ -189,21 +189,25 @@ define CFG_MAKE_TOOLCHAIN
   endif
 
   CFG_COMPILE_C_$(1) = $$(CC_$(1)) \
+        $$(CFLAGS) \
         $$(CFG_GCCISH_CFLAGS) \
         $$(CFG_GCCISH_CFLAGS_$(1)) \
         -c $$(call CFG_CC_OUTPUT_$(1),$$(1)) $$(2)
   CFG_LINK_C_$(1) = $$(CC_$(1)) \
+        $$(LDFLAGS) \
         $$(CFG_GCCISH_LINK_FLAGS) -o $$(1) \
         $$(CFG_GCCISH_LINK_FLAGS_$(1)) \
         $$(CFG_GCCISH_DEF_FLAG_$(1))$$(3) $$(2) \
         $$(call CFG_INSTALL_NAME_$(1),$$(4))
   CFG_COMPILE_CXX_$(1) = $$(CXX_$(1)) \
+        $$(CXXFLAGS) \
         $$(CFG_GCCISH_CFLAGS) \
         $$(CFG_GCCISH_CXXFLAGS) \
         $$(CFG_GCCISH_CFLAGS_$(1)) \
         $$(CFG_GCCISH_CXXFLAGS_$(1)) \
         -c $$(call CFG_CC_OUTPUT_$(1),$$(1)) $$(2)
   CFG_LINK_CXX_$(1) = $$(CXX_$(1)) \
+        $$(LDFLAGS) \
         $$(CFG_GCCISH_LINK_FLAGS) -o $$(1) \
         $$(CFG_GCCISH_LINK_FLAGS_$(1)) \
         $$(CFG_GCCISH_DEF_FLAG_$(1))$$(3) $$(2) \