Having hard time adding to CFLAGS

[Denys Vlasenko <vda.linux@googlemail.com>]

Let's say I need to add a gcc option to my musl build.

configure says:
...
Some influential environment variables:
  CC                      C compiler command [detected]
  CFLAGS                  C compiler flags [-Os -pipe ...]
  CROSS_COMPILE           prefix for cross compiler and tools [none]
  LIBCC                   compiler runtime library [detected

So I try this, combining all possible ways of passing CFLAGS
(past experience is that different projects do it differently).

CFLAGS is in environment, and on both configure and make
command lines:

export CFLAGS="-falign-functions=1"    # for example
./configure CFLAGS="$CFLAGS"
make CFLAGS="$CFLAGS"

It does work, but resulting libc.so is twice as big:
   text       data        bss        dec        hex    filename
 564099       1944      11768     577811      8d113    musl.1/lib/libc.so
 917805       2130      11736     931671      e3757    musl.2/lib/libc.so

The cause is that gcc invocation for each .c file in both cases start normally:

gcc -std=c99 -nostdinc -ffreestanding -fexcess-precision=standard
-frounding-math -D_XOPEN_SOURCE=700 -I./arch/x86_64 -I./src/internal
-I./include...

but then, build without explicit CFLAGS use this:

... -Os -pipe -fomit-frame-pointer -fno-unwind-tables
-fno-asynchronous-unwind-tables -Wa,--noexecstack
-Werror=implicit-function-declaration -Werror=implicit-int
-Werror=pointer-sign -Werror=pointer-arith -include vis.h  -fPIC
-DSHARED -c -o src/aio/aio.lo src/aio/aio.c

and one with CFLAGS loses these flags, in particular, it has no -Os
and no -fPIC:

... -falign-functions=1 -c -o src/aio/aio.o src/aio/aio.c

Evidently, my CFLAGS replaced needed flags instead of being added at the end.

Can this be fixed? If user needs to use e.g. EXTRA_CFLAGS instead,
please fix configure --help.