Make poll() the default (#2065)

* Make poll() the default

select() can still be enabled by defining CPPHTTPLIB_USE_SELECT.

* Run tests with select() and poll()

test/Makefile

@@ -1,6 +1,10 @@
 CXX = clang++
 CXXFLAGS = -g -std=c++11 -I. -Wall -Wextra -Wtype-limits -Wconversion -Wshadow # -fno-exceptions -DCPPHTTPLIB_NO_EXCEPTIONS -fsanitize=address
 
+ifeq ($(SELECT_IMPL),select)
+    CXXFLAGS += -DCPPHTTPLIB_USE_SELECT
+endif
+
 PREFIX ?= $(shell brew --prefix)
 
 OPENSSL_DIR = $(PREFIX)/opt/openssl@3