[Bug build/21234] New: use of CFLAGS makes glibc detect no optimization

["hlfqdhj at 163 dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=21234

            Bug ID: 21234
           Summary: use of CFLAGS makes glibc detect no optimization
           Product: glibc
           Version: 2.25
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: build
          Assignee: unassigned at sourceware dot org
          Reporter: hlfqdhj at 163 dot com
                CC: carlos at redhat dot com
  Target Milestone: ---

Hi there,

Sorry if this is a duplicate.

I was trying to build glibc with CC set to '/usr/bin/gcc', CFLAGS set as
'-march=native' and --disable-sanity-check (since I'd like it to be installed
to /usr/local). It fails nearly immediately, saying that glibc cannot be
compiled without optimization.

Later, I attempt to try with CC set directly as '-march=native -O2' (It seems
that CFLAGS is sorted), and it compiles until vprintf.c, complaining:

In file included from ../bits/libc-header-start.h:33:0,
                 from ../libio/stdio.h:28,
                 from ../include/stdio.h:5,
                 from vprintf.c:20:
../include/features.h:373:4: error: #warning _FORTIFY_SOURCE requires compiling
with optimization (-O) [-Werror=cpp]
 #  warning _FORTIFY_SOURCE requires compiling with optimization (-O)

And I have seen explicit two -O2 from 

/usr/bin/gcc -march=native -O2 vprintf.c -c -std=gnu11 -fgnu89-inline  -O2
-Wall -Werror -Wundef -Wwrite-strings -fmerge-all-constants
-fno-stack-protector -frounding-math -fstack-protector-strong -march=x86-64
-mtune=generic -pipe -Wstrict-prototypes -Wold-style-definition   -fexceptions 
-ftls-model=initial-exec   -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2  -I../include
(MUCH MORE...)

The preprocessor seems fail to detect optimization.

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