Makefile: Remove misleading comments on BASE_*FLAGS

I added these comments attempting to describe the difference between
BASE_CFLAGS and CFLAGS.  However, the description given isn't really
accurate, and I'm not even sure the concept I'm trying to describe is
coherent.  Just remove it.

Suggested-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Author: dgibson

Makefile

@@ -30,15 +30,11 @@ ifeq ($(shell $(CC) -O2 -dM -E - < /dev/null 2>&1 | grep ' _FORTIFY_SOURCE ' > /
 FORTIFY_FLAG := -D_FORTIFY_SOURCE=2
 endif
 
-# Mandatory preprocessor flags that won't be overridden with $(CPPFLAGS)
-# FIXME: Could some of these be default, rather than required?
 BASE_CPPFLAGS := -D_XOPEN_SOURCE=700 -D_GNU_SOURCE $(FORTIFY_FLAG)
 BASE_CPPFLAGS += -DPAGE_SIZE=$(shell getconf PAGE_SIZE)
 BASE_CPPFLAGS += -DVERSION=\"$(VERSION)\"
 BASE_CPPFLAGS += -DDUAL_STACK_SOCKETS=$(DUAL_STACK_SOCKETS)
 
-# Mandatory compiler flags that won't be overridden with $(CFLAGS)
-# FIXME: Could some of these be default, rather than required?
 BASE_CFLAGS := -std=c11 -pie -fPIE -O2
 BASE_CFLAGS += -pedantic -Wall -Wextra -Wno-format-zero-length -Wformat-security