Makefile: use phony targets

Add 'build', 'build-*', and 'install' as phony targets to the top level
Makefile.

I tend to do everything in tree with the build in a directory called
'build'. This interfered with the target 'build', making it think the
target was up to date, and stopping scripts/crosstool-NG.sh from being
regenerated.

Added 'install' as I often set the prefix to $PWD/install.

Signed-off-by: Michael Hope <michael.hope@linaro.org>

--- 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 (;;)