Meson updates (#1124)

* Modernize meson.build

* Make tests optional
* Use `files()` for quick sanity checks

* Bump version to 1.9.3

* Bump SOVERSION, as some functions were removed
  and structs were changed, as determined by
  libabigail.
diff --git a/include/json/version.h b/include/json/version.h
index ff94372..0f29834 100644
--- a/include/json/version.h
+++ b/include/json/version.h
@@ -9,10 +9,10 @@
 // 3. /CMakeLists.txt
 // IMPORTANT: also update the SOVERSION!!
 
-#define JSONCPP_VERSION_STRING "1.9.2"
+#define JSONCPP_VERSION_STRING "1.9.3"
 #define JSONCPP_VERSION_MAJOR 1
 #define JSONCPP_VERSION_MINOR 9
-#define JSONCPP_VERSION_PATCH 2
+#define JSONCPP_VERSION_PATCH 3
 #define JSONCPP_VERSION_QUALIFIER
 #define JSONCPP_VERSION_HEXA                                                   \
   ((JSONCPP_VERSION_MAJOR << 24) | (JSONCPP_VERSION_MINOR << 16) |             \