build: Only build examples if nghttp3 is present

Makefile.am

@@ -20,7 +20,11 @@
 # LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
 # OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
 # WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
-SUBDIRS = lib tests third-party examples
+SUBDIRS = lib tests
+
+if HAVE_NGHTTP3
+SUBDIRS += third-party examples
+endif
 
 ACLOCAL_AMFLAGS = -I m4
 

configure.ac

@@ -139,6 +139,7 @@ PKG_CHECK_MODULES([LIBNGHTTP3], [libnghttp3 >= 0.0.0],
 if test "${have_libnghttp3}" = "xno"; then
   AC_MSG_NOTICE($LIBNGHTTP3_PKG_ERRORS)
 fi
+AM_CONDITIONAL([HAVE_NGHTTP3], [ test "x${have_libnghttp3}" = "xyes" ])
 
 # libev (for examples)
 # libev does not have pkg-config file.  Check it in an old way.