Making libfst a proper gitsubmodule

Author: pieter3d

.gitmodules

@@ -8,3 +8,6 @@
 [submodule "external/slang"]
 	path = external/slang
 	url = https://github.com/MikePopoloski/slang.git
+[submodule "external/libfst"]
+	path = external/libfst
+	url = https://github.com/gtkwave/libfst.git

external/CMakeLists.txt

@@ -22,4 +22,13 @@ add_subdirectory(abseil-cpp EXCLUDE_FROM_ALL)
 add_subdirectory(googletest EXCLUDE_FROM_ALL)
 include(GoogleTest)
 add_subdirectory(slang EXCLUDE_FROM_ALL)
-add_subdirectory(fst)
+
+# libfst doesn't have its own CMakeLists.txt, so just define the libraries for it here.
+find_package(ZLIB REQUIRED)
+add_library(libfst
+  libfst/src/fstapi.c
+  libfst/src/fastlz.c
+  libfst/src/lz4.c
+)
+target_link_libraries(libfst PRIVATE z)
+target_include_directories(libfst PRIVATE libfst/src)