build: enable meson build

Add meson build support.  Autotools support will be removed in
the future once the consuming meta-phosphor recipes are updated.

This should be feature complete relative to the previous autotools
support.

Additionally, support is added for using sdbusplus as a subproject
in the event that users do not have sdbusplus installed.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ic2c0129bda11cf10aaa557c2fd6403e2ed2cd8dd
diff --git a/.gitignore b/.gitignore
index aaaec40..92622d3 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,6 @@
+build*/
+subprojects/*/
+
 # Template from:
 # https://github.com/github/gitignore/blob/master/Autotools.gitignore