COMP: Reorganize cmake files

ext/peglib.cmake

@@ -0,0 +1,20 @@
+include(FetchContent)
+
+FetchContent_Declare(
+    peglib
+    GIT_REPOSITORY https://github.com/yhirose/cpp-peglib
+    GIT_TAG v1.8.2
+    GIT_PROGRESS TRUE
+)
+
+set(PEGLIB_BUILD_TESTS OFF CACHE INTERNAL "")
+
+FetchContent_GetProperties(peglib)
+if(NOT ${peglib}_POPULATED)
+    FetchContent_Populate(peglib)
+    find_package(Threads)
+    add_library(peglib INTERFACE ${peglib_SOURCE_DIR}/peglib.h)
+    target_include_directories(peglib INTERFACE ${peglib_SOURCE_DIR})
+    target_link_libraries(peglib INTERFACE Threads::Threads)
+    add_library(peglib::peglib ALIAS peglib)
+endif()