configure: Set default HOSTCFLAGS/HOSTCPPFLAGS after compiler detection

This prevents the default HOSTCFLAGS from getting clobbered by flags
passed to configure on the command line.

configure

@@ -2125,8 +2125,6 @@ HOSTCC_E='-E -o $@'
 HOSTCC_O='-o $@'
 HOSTLD_O='-o $@'
 
-host_cflags='-O3'
-host_cppflags='-D_ISOC99_SOURCE'
 host_libs='-lm'
 host_cflags_filter=echo
 host_ldflags_filter=echo
@@ -3167,8 +3165,10 @@ check_cc -D_LARGEFILE_SOURCE <<EOF && add_cppflags -D_LARGEFILE_SOURCE
 #include <stdlib.h>
 EOF
 
+add_host_cppflags -D_ISOC99_SOURCE
 check_host_cflags -std=c99
 check_host_cflags -Wall
+check_host_cflags -O3
 
 check_64bit(){
     arch32=$1