[PATCH] configure.ac: fix build on powerpc

[Fabrice Fontaine]

Fix the following powerpc build failures on:
 - musl raised because fpu_control.h is not available:

In file included from fp.c:8:
fp-gnuppc.c:21:10: fatal error: fpu_control.h: No such file or directory
   21 | #include <fpu_control.h>
      |          ^~~~~~~~~~~~~~~

 - glibc raised because _FPU_RC_NEAREST is undefined if _SOFT_FLOAT is
   set:

   In file included from fp.c:8:
fp-gnuppc.c: In function 'gsl_ieee_set_mode':
fp-gnuppc.c:53:15: error: '_FPU_RC_NEAREST' undeclared (first use in this 
function)
   53 |       mode |= _FPU_RC_NEAREST ;
      |               ^~~~~~~~~~~~~~~

Fixes:
 - 
http://autobuild.buildroot.org/results/d73e116c81bf16d2e55fced215d6bd3b382fef10
 - 
http://autobuild.buildroot.org/results/48403946bb4cda9013e51db59c1b2ffdcf4e2854

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
---
 configure.ac | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/configure.ac b/configure.ac
index bfd9ca30..1ee6dca0 100644
--- a/configure.ac
+++ b/configure.ac
@@ -389,6 +389,14 @@ if test "$ac_cv_c_ieee_interface" = "gnux86" ; then
    fi
 fi
 
+if test "$ac_cv_c_ieee_interface" = "gnuppc" ; then
+    AC_CACHE_CHECK([for _FPU_RC_NEAREST], ac_cv_c__fpu_rc_nearest,
+    [ac_cv_c__fpu_rc_nearest=no
+    AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <fpu_control.h>
+]], [[ unsigned short mode = _FPU_RC_NEAREST ; 
]])],[ac_cv_c__fpu_rc_nearest="yes"],[ac_cv_c_ieee_interface=unknown])
+    ])
+fi
+
 ac_tr_ieee_interface=HAVE_`echo $ac_cv_c_ieee_interface | tr 
'abcdefghijklmnopqrstuvwxyz' 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'`_IEEE_INTERFACE
 AC_DEFINE_UNQUOTED($ac_tr_ieee_interface,1,[IEEE Interface Type])
 
-- 
2.35.1