build: Add missing m4 quotes to sed regex

This was making the regex non-functional, as the square brackets
were being removed when generating the output file, thus making the
--disable-compiler-optimisations and --disable-linker-optimisations
non-functional.

m4/dpkg-compiler.m4

@@ -36,7 +36,7 @@ AC_DEFUN([DPKG_COMPILER_OPTIMISATIONS],
 	AS_HELP_STRING([--disable-compiler-optimisations],
 		       [Disable compiler optimisations]),
 [if test "x$enable_compiler_optimisations" = "xno"; then
-	[CFLAGS=$(echo "$CFLAGS" | sed -e "s/ -O[1-9]*\b/ -O0/g")]
+	[CFLAGS=$(echo "$CFLAGS" | sed -e "s/ -O[[1-9]]*\b/ -O0/g")]
 fi])dnl
 ])
 

m4/dpkg-linker.m4

@@ -8,7 +8,7 @@ AC_DEFUN([DPKG_LINKER_OPTIMISATIONS],
 	AS_HELP_STRING([--disable-linker-optimisations],
 		       [Disable linker optimisations]),
 [if test "x$enable_linker_optimisations" = "xno"; then
-	[LDFLAGS=$(echo "$LDFLAGS" | sed -e "s/ -Wl,-O[0-9]*\b//g")]
+	[LDFLAGS=$(echo "$LDFLAGS" | sed -e "s/ -Wl,-O[[0-9]]*\b//g")]
 else
 	[LDFLAGS="$LDFLAGS -Wl,-O1"]
 fi], [LDFLAGS="$LDFLAGS -Wl,-O1"])dnl