fix issue oxfordcontrol#46 - support use via add_subdirectory

This commit allows Clarabel.cpp to be consumed by other CMake projects
via a call to `add_subdirectory`:

```cmake

add_subdirectory(path/to/clarabel.cpp)
```

Other projects might have Clarabel.cpp as a git submodule, and this
makes it easier to do so.

CMakeLists.txt

@@ -12,11 +12,16 @@ set(CMAKE_CXX_STANDARD_REQUIRED True)
 set(CMAKE_C_STANDARD 11)
 set(CMAKE_C_STANDARD_REQUIRED True)
 
+# Set the root directory of the project. This is needed
+# so that Clarabel.cpp can be consumed by other projects via
+# `add_subdirectory`
+set(CLARABEL_ROOT_DIR "${CMAKE_CURRENT_SOURCE_DIR}")
+
 #----------------------------------------------
 # Clarabel feature configuration
 #----------------------------------------------
 set(CLARABEL_FEATURE_SDP "none" CACHE STRING "Package for SDP to be selected")
-set_property(CACHE CLARABEL_FEATURE_SDP PROPERTY STRINGS 
+set_property(CACHE CLARABEL_FEATURE_SDP PROPERTY STRINGS
  none
  sdp-accelerate
  sdp-netlib
@@ -55,4 +60,4 @@ option(CLARABEL_BUILD_TESTS "Build the unit tests for Clarabel.cpp" false)
 if(CLARABEL_BUILD_TESTS)
  enable_testing()
  add_subdirectory(tests)
-endif()
+endif()

rust_wrapper/CMakeLists.txt

@@ -10,10 +10,10 @@ add_library(libclarabel_c_shared INTERFACE)
 # Debug/Release flags
 if(CMAKE_BUILD_TYPE MATCHES Release)
  set(clarabel_c_build_flags "--release")
- set(clarabel_c_output_directory "${CMAKE_SOURCE_DIR}/rust_wrapper/target/release")
+ set(clarabel_c_output_directory "${CLARABEL_ROOT_DIR}/rust_wrapper/target/release")
 else()
  set(clarabel_c_build_flags "")
- set(clarabel_c_output_directory "${CMAKE_SOURCE_DIR}/rust_wrapper/target/debug")
+ set(clarabel_c_output_directory "${CLARABEL_ROOT_DIR}/rust_wrapper/target/debug")
 endif()
 
 set(CLARABEL_C_OUTPUT_DIR ${clarabel_c_output_directory} PARENT_SCOPE)
@@ -86,7 +86,7 @@ endif()
 # Add the cargo project as a custom target
 add_custom_target(
  libclarabel_c
- WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}/rust_wrapper
+ WORKING_DIRECTORY ${CLARABEL_ROOT_DIR}/rust_wrapper
  # Commands for building the Rust library
  COMMAND cargo build ${clarabel_c_build_flags}
  COMMAND cargo install cbindgen --version 0.24.5
@@ -104,10 +104,10 @@ add_custom_target(
 
 # Static library
 target_link_libraries(libclarabel_c_static INTERFACE ${LIBCLARABEL_C_STATIC_PATH})
-target_include_directories(libclarabel_c_static INTERFACE ${CMAKE_SOURCE_DIR}/include)
+target_include_directories(libclarabel_c_static INTERFACE ${CLARABEL_ROOT_DIR}/include)
 add_dependencies(libclarabel_c_static libclarabel_c)
 
 # Shared library
 target_link_libraries(libclarabel_c_shared INTERFACE ${LIBCLARABEL_C_SHARED_PATH})
-target_include_directories(libclarabel_c_shared INTERFACE ${CMAKE_SOURCE_DIR}/include)
+target_include_directories(libclarabel_c_shared INTERFACE ${CLARABEL_ROOT_DIR}/include)
 add_dependencies(libclarabel_c_shared libclarabel_c)