Add meson build system

Meson is a build system somewhat like cmake, but without all of the
rough edges. It supports many OSes, including all of the major ones, and
a large number of C++ compilers.

My interest isn't really in convincing people to use meson as the
default here, but meson provides a subproject mechanism that can fetch
external projects and build them along with the main project in a single
configure/compile invocation. This is extremely useful for platforms
that lack a (competent) package manager.

As far as I know the meson build does everything the cmake build does,
with one exception: generate the cmake config/version files. meson can
generate these files, but only in simple cases, and not when using
export targets like tinyxml2 does.

setversion.py

@@ -134,6 +134,15 @@ def cmakeRule2( line ):
 
 fileProcess( "CMakeLists.txt", cmakeRule2 )
 
+def mesonRule(line):
+	match = re.search(r"(\s*version) : '(\d+.\d+.\d+)',", line)
+	if match:
+		print("1)meson.build version found.")
+		return "{} : '{}.{}.{}',\n".format(match.group(1), major, minor, build)
+	return line
+
+fileProcess("meson.build", mesonRule)
+
 print( "Release note:" )
 print( '1. Build.   g++ -Wall -DTINYXML2_DEBUG tinyxml2.cpp xmltest.cpp -o gccxmltest.exe' )
 print( '2. Commit.  git commit -am"setting the version to ' + versionStr + '"' )