package/glibc: can't be built with Optimize for fast

Fix the following build failure:

In file included from <command-line>:
./../include/libc-symbols.h:82:3: error: #error "glibc must not be compiled with -ffast-math"
   82 | # error "glibc must not be compiled with -ffast-math"
      |   ^~~~~

Fixes:
 - http://autobuild.buildroot.org/results/3d91f3685ae6c3e1f765d419ea2c0da4cb0eb3be

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/glibc/glibc.mk

@@ -118,6 +118,11 @@ ifeq ($(BR2_nios2),y)
 GLIBC_CFLAGS += -O2
 endif
 
+# glibc can't be built with Optimize for fast
+ifeq ($(BR2_OPTIMIZE_FAST),y)
+GLIBC_CFLAGS += -O2
+endif
+
 define GLIBC_CONFIGURE_CMDS
 	mkdir -p $(@D)/build
 	# Do the configuration