[M4] add exec::run subprocess wrapper

CMakeLists.txt

@@ -30,6 +30,7 @@ if(CARGOXX_WERROR)
 endif()
 
 find_package(SQLite3 REQUIRED)
+find_package(reproc REQUIRED)
 
 # ----- cargoxx library: module units + implementation units -----
 add_library(cargoxx STATIC)
@@ -50,6 +51,7 @@ target_sources(cargoxx
         src/linkdb/overlay.cpp
         src/codegen/flake.cpp
         src/codegen/cmake.cpp
+        src/exec/subprocess.cpp
         src/cli/cmd_new.cpp
         src/cli/cmd_build.cpp
         src/cli/run.cpp
@@ -67,7 +69,7 @@ target_sources(cargoxx
             src/cli/cli.cppm
 )
 
-target_link_libraries(cargoxx PRIVATE SQLite::SQLite3)
+target_link_libraries(cargoxx PRIVATE SQLite::SQLite3 reproc)
 
 # ----- cargoxx binary -----
 add_executable(cargoxx_bin src/main.cpp)