Added basic CMake support (#66)

This will enable package managers and projects that relies on CMake,
like [CPM](https://github.com/cpm-cmake/CPM.cmake) to add SimpleIni
as a dependency easily. Note that it does not enable building the test
harness using CMake, this is still via the Makefile.

Signed-off-by: Vinícius Ferrão <[email protected]>

CMakeLists.txt

@@ -0,0 +1,37 @@
+cmake_minimum_required(VERSION 3.14)
+
+project(
+	SimpleIni
+	VERSION 4.20
+	DESCRIPTION "Cross-platform C++ library providing a simple API to read and write INI-style configuration files"
+	LANGUAGES CXX
+)
+
+if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME)
+	set(IS_TOPLEVEL_PROJECT TRUE)
+else()
+	set(IS_TOPLEVEL_PROJECT FALSE)
+endif()
+
+# Disable in-source builds:
+get_filename_component(srcdir "${CMAKE_SOURCE_DIR}" REALPATH)
+get_filename_component(bindir "${CMAKE_BINARY_DIR}" REALPATH)
+
+if("${srcdir}" STREQUAL "${bindir}")
+	message("")
+	message("ERROR:: in-source builds are disabled!")
+	message("Run cmake in a separate build directory:")
+	message("$ cmake -S . -B build")
+	message("")
+	message(FATAL_ERROR "Aborting...")
+endif()
+
+# Define library paths and include directories
+set(EXPORT_NAMESPACE "${PROJECT_NAME}::")
+set(HEADERS SimpleIni.h)
+
+add_library(${PROJECT_NAME} INTERFACE)
+add_library(${EXPORT_NAMESPACE}${PROJECT_NAME} ALIAS ${PROJECT_NAME})
+
+target_include_directories(${PROJECT_NAME} INTERFACE .)
+