feat: [Makefile] Support g++ min version, cygwin

Update README.

Author: manid2

README.md

@@ -49,9 +49,15 @@ Coding guidelines
 Build & test
 ------------
 
+This repository requires Gnu g++ compiler minimum version 13 to support C++20
+features such as `std::format()` used in code.
+
 This repository uses Gnu Makefile to run build and test tasks along with some
 helper tasks such as to run formatter, linter for source code.
 
+Build is tested and tests are verified on Gnu/Linux & Cygwin (Windows)
+operating systems.
+
 ### Examples
 
 ```bash

cpp/Makefile

@@ -8,9 +8,25 @@ all: tests check
 # ============================================================================
 include ../mk/common.mk
 
-CXX = /usr/bin/g++-13
+CXX = /usr/bin/g++
+MIN_GPP_VERSION=13
+GPP_VERSION:=$(shell $(CXX) -dumpfullversion | cut -d. -f1)
+CHECK_GPP_VERSION:=$(shell expr "$(GPP_VERSION)" ">=" "$(MIN_GPP_VERSION)")
+
+ifneq "$(CHECK_GPP_VERSION)" "1"
+  $(error ERROR: $(CXX) version $(GPP_VERSION) is lower than required \
+          minimum version $(MIN_GPP_VERSION))
+endif
+
 CXXFLAGS = -std=c++20 -Wall -Wextra -Wpedantic -Werror -Iincludes
 
+SYS_NAME:=$(shell uname -s)
+IS_CYGWIN:=$(findstring CYGWIN, $(SYS_NAME))
+
+ifeq ($(IS_CYGWIN), CYGWIN)
+  CXXFLAGS += -static-libstdc++
+endif
+
 BUILD_DIR:=build
 
 HDRS:=$(shell $(GIT_LS) -- 'includes/*.h')