Change GCC mode from ISO C90 to ISO C99

Add GCC flag "-std=c99".  Also remove GCC flag "-ansi" which is an
alias for "-std=c90" and would make the flag "-std=c99" ineffective.
This fixes all occurrences of the following GCC warning:

warning: ISO C90 does not support '__func__' predefined identifier

acinclude.m4

@@ -27,9 +27,11 @@ AC_DEFUN([SANE_SET_CFLAGS],
 [
 if test "${ac_cv_c_compiler_gnu}" = "yes"; then
   NORMAL_CFLAGS="\
+      -std=c99 \
       -W \
       -Wall"
   WARN_CFLAGS="\
+      -std=c99 \
       -W \
       -Wall \
       -Wcast-align \
@@ -41,16 +43,6 @@ if test "${ac_cv_c_compiler_gnu}" = "yes"; then
       -Wstrict-prototypes \
       -pedantic"
 
-  # Some platforms are overly strict with -ansi enabled.  Exclude those.
-  ANSI_FLAG=-ansi
-  case "${host_os}" in  
-    solaris* | hpux* | os2* | darwin* | cygwin* | mingw*)
-      ANSI_FLAG=
-      ;;
-  esac
-  NORMAL_CFLAGS="${NORMAL_CFLAGS} ${ANSI_FLAG}"
-  WARN_CFLAGS="${WARN_CFLAGS} ${ANSI_FLAG}"
-
   AC_ARG_ENABLE(warnings,
     AC_HELP_STRING([--enable-warnings],
                    [turn on tons of compiler warnings (GCC only)]),

configure

@@ -5185,9 +5185,11 @@ fi
 
 if test "${ac_cv_c_compiler_gnu}" = "yes"; then
   NORMAL_CFLAGS="\
+      -std=c99 \
       -W \
       -Wall"
   WARN_CFLAGS="\
+      -std=c99 \
       -W \
       -Wall \
       -Wcast-align \
@@ -5199,16 +5201,6 @@ if test "${ac_cv_c_compiler_gnu}" = "yes"; then
       -Wstrict-prototypes \
       -pedantic"
 
-  # Some platforms are overly strict with -ansi enabled.  Exclude those.
-  ANSI_FLAG=-ansi
-  case "${host_os}" in
-    solaris* | hpux* | os2* | darwin* | cygwin* | mingw*)
-      ANSI_FLAG=
-      ;;
-  esac
-  NORMAL_CFLAGS="${NORMAL_CFLAGS} ${ANSI_FLAG}"
-  WARN_CFLAGS="${WARN_CFLAGS} ${ANSI_FLAG}"
-
   # Check whether --enable-warnings was given.
 if test "${enable_warnings+set}" = set; then :
   enableval=$enable_warnings;