Adjust to unbundle a few extras.

zstd changes only needed because of https://bugs.gentoo.org/872254
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -54,3 +54,4 @@
 	set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
-	add_subdirectory(3rdparty/gtest EXCLUDE_FROM_ALL)
+	find_package(GTest REQUIRED)
+	alias_library(gtest_main GTest::gtest_main)
 	add_subdirectory(tests/ctest)
--- a/cmake/SearchForStuff.cmake
+++ b/cmake/SearchForStuff.cmake
@@ -177,3 +177,3 @@
 set(ACTUALLY_ENABLE_TESTS ${ENABLE_TESTS})
-if(ENABLE_TESTS)
+if(0)
 	if(NOT EXISTS "${CMAKE_SOURCE_DIR}/3rdparty/gtest/CMakeLists.txt")
@@ -210,7 +210,4 @@
 find_optional_system_library(ryml 3rdparty/rapidyaml/rapidyaml 0.4.0)
-find_optional_system_library(zstd 3rdparty/zstd 1.4.5)
-if (${zstd_TYPE} STREQUAL System)
-	alias_library(Zstd::Zstd zstd::libzstd_shared)
-	alias_library(pcsx2-zstd zstd::libzstd_shared)
-endif()
+pkg_check_modules(zstd REQUIRED IMPORTED_TARGET libzstd)
+alias_library(Zstd::Zstd PkgConfig::zstd)
 find_optional_system_library(libzip 3rdparty/libzip 1.8.0)
@@ -243,3 +240,4 @@
 add_subdirectory(3rdparty/lzma EXCLUDE_FROM_ALL)
-add_subdirectory(3rdparty/libchdr EXCLUDE_FROM_ALL)
+pkg_check_modules(chdr REQUIRED IMPORTED_TARGET libchdr)
+alias_library(chdr-static PkgConfig::chdr)
 
@@ -269,5 +267,3 @@
 if(CUBEB_API)
-	add_subdirectory(3rdparty/cubeb EXCLUDE_FROM_ALL)
-	target_compile_options(cubeb PRIVATE "-w")
-	target_compile_options(speex PRIVATE "-w")
+	find_package(cubeb REQUIRED)
 endif()