scripts: add BUILD/HOST extra cflags/ldflags

On some hosts, and for certain toolchains (eg. toolchain targetting
the upcoming Darwin), it may be necessary to pass arbitrary CFLAGS
and/or LDFLAGS when building the components.

And necessary infrastructure:
- EXTRA_{CFLAGS,LDFLAGS}_FOR_{BUILD,HOST} as config options
- pass those extra flags to components

Fix-up a slight typo in elf2flt at the same time (misnamed cflags).

Signed-off-by: Yann Diorcet <diorcet.yann@gmail.com>
Message-Id: <d24043276c9243a35421.1353077450@macbook-smorlat.local>
Patchwork-Id: 199645

 --- trunk/src/acos.c	2009/12/08 16:49:43	733

 +++ trunk/src/acos.c	2009/12/09 20:51:38	734

 @@ -188,10 +188,7 @@

    else

      rnd_im = rnd_im == GMP_RNDU ? GMP_RNDD

        : rnd_im == GMP_RNDD ? GMP_RNDU

 -#if MPFR_VERSION_MAJOR >= 3

 -      : rnd_im == GMP_RNDA ? GMP_RNDZ

 -#endif

 -      : rnd_im;

 +      : rnd_im; /* both RNDZ and RNDA map to themselves for -asin(z) */

    rnd1 = RNDC(GMP_RNDN, rnd_im);

    mpfr_init2 (pi_over_2, p);

    for (;;)