configure.ac: Summarize how many times we warned.

It's way too easy to have a warning in the middle of the configure
process that the user never notices.

configure.ac

@@ -24,6 +24,11 @@ AC_DEFINE(APPROX_RELEASE_DATE, ["2020-06-09"], # for 0.4.5.0-alpha-dev
 # "subdir-objects" means put .o files in the same directory as the .c files
 AM_INIT_AUTOMAKE([foreign 1.11 subdir-objects -Wall -Werror])
 
+tor_ac_n_warnings=0
+tor_incr_n_warnings() {
+  tor_ac_n_warnings=`expr $tor_ac_n_warnings + 1`
+}
+
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 AC_CONFIG_HEADERS([orconfig.h])
 
@@ -432,6 +437,7 @@ AC_SUBST([PYTHON], [$tor_cv_PYTHON])
 PYTHON="$tor_cv_PYTHON"
 
 if test "x$PYTHON" = "x"; then
+  tor_incr_n_warnings
   AC_MSG_WARN([Python 3 unavailable; some tests will not be run.])
 fi
 
@@ -1229,6 +1235,7 @@ else
                       have_lzma=no)
 
     if test "x$have_lzma" = "xno" ; then
+        tor_incr_n_warnings
         AC_MSG_WARN([Unable to find liblzma, $pkg_config_user_action, or set LZMA_CFLAGS and LZMA_LIBS.])
     fi
 fi
@@ -1261,6 +1268,7 @@ else
                       have_zstd=no)
 
     if test "x$have_zstd" = "xno" ; then
+        tor_incr_n_warnings
         AC_MSG_WARN([Unable to find libzstd, $pkg_config_user_action, or set ZSTD_CFLAGS and ZSTD_LIBS.])
     fi
 fi
@@ -1365,6 +1373,7 @@ fi
 if test "$fragile_hardening" = "yes"; then
     TOR_TRY_COMPILE_WITH_CFLAGS(-ftrapv, also_link, CFLAGS_FTRAPV="-ftrapv", true)
    if test "$tor_cv_cflags__ftrapv" = "yes" && test "$tor_can_link__ftrapv" != "yes"; then
+      tor_incr_n_warnings
       AC_MSG_WARN([The compiler supports -ftrapv, but for some reason I was not able to link with -ftrapv. Are you missing run-time support? Run-time hardening will not work as well as it should.])
    fi
 
@@ -1905,6 +1914,7 @@ void try_atomic_init(struct x *xx)
 if test "$tor_cv_stdatomic_works" = "yes"; then
    AC_DEFINE(STDATOMIC_WORKS, 1, [Set to 1 if we can compile a simple stdatomic example.])
 elif test "$ac_cv_header_stdatomic_h" = "yes"; then
+   tor_incr_n_warnings
    AC_MSG_WARN([Your compiler provides the stdatomic.h header, but it doesn't seem to work.  I'll pretend it isn't there. If you are using Clang on Debian, maybe this is because of https://bugs.debian.org/903709 ])
 fi
 
@@ -2088,6 +2098,7 @@ AS_CASE([$malloc],
   ],
 
   [openbsd], [
+    tor_incr_n_warnings
     AC_MSG_WARN([The openbsd malloc port is deprecated in Tor 0.3.5 and will be removed in a future version.])
     enable_openbsd_malloc=yes
   ],
@@ -2616,6 +2627,7 @@ TOR_TRY_COMPILE_WITH_CFLAGS([@warning_flags], [],
 if test "$enable_coverage" = "yes" && test "$have_clang" = "no"; then
    case "$host_os" in
     darwin*)
+      tor_incr_n_warnings
       AC_MSG_WARN([Tried to enable coverage on OSX without using the clang compiler. This might not work! If coverage fails, use CC=clang when configuring with --enable-coverage.])
    esac
 fi
@@ -2653,6 +2665,7 @@ if test "x$asciidoc" = "xtrue" && test "$ASCIIDOC" = "none"; then
 fi
 
 if test "$fragile_hardening" = "yes"; then
+  tor_incr_n_warnings
   AC_MSG_WARN([
 
 ============
@@ -2670,6 +2683,7 @@ fi
 AC_OUTPUT
 
 if test "$openssl_ver_mismatch" = "yes"; then
+   tor_incr_n_warnings
    AC_MSG_WARN([
 ============
 Warning! The version OpenSSL headers we get from compiling with
@@ -2831,3 +2845,10 @@ PPRINT_PROP_STRING([Man Pages], [$report_mandir])
 
 AS_ECHO
 AS_ECHO(["Configure Line: ./configure $configure_flags"])
+
+if test "$tor_ac_n_warnings" != "0"; then
+  AS_ECHO
+  PPRINT_WARN([
+Encountered $tor_ac_n_warnings warning(s). See messages above for more info.
+  ])
+fi