Forward all potentially relevant CMake flags to ExternalProject

Previously, flags such as CMAKE_SYSROOT and CMAKE_C_FLAGS_INIT were not
forwarded to ExternalProject, which lead to subtle build issues with
Bazel's rules_foreign_cc.

This commit also ensures that all C/CXX flags are used for the 32 bit
build. Previously, these flags were overridden by the single flag
`-m32`.

CMakeLists.txt

@@ -24,11 +24,20 @@ option(TARGET_32BIT "Make the compiler work correctly with -m32" OFF)
 include(ExternalProject)
 
 set(SYM_RUNTIME_BUILD_ARGS
+  -DCMAKE_AR=${CMAKE_AR}
   -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
   -DCMAKE_C_FLAGS=${CMAKE_C_FLAGS}
+  -DCMAKE_C_FLAGS_INIT=${CMAKE_C_FLAGS_INIT}
   -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
   -DCMAKE_CXX_FLAGS=${CMAKE_CXX_FLAGS}
+  -DCMAKE_CXX_FLAGS_INIT=${CMAKE_CXX_FLAGS_INIT}
+  -DCMAKE_EXE_LINKER_FLAGS=${CMAKE_EXE_LINKER_FLAGS}
+  -DCMAKE_EXE_LINKER_FLAGS_INIT=${CMAKE_EXE_LINKER_FLAGS_INIT}
+  -DCMAKE_MODULE_LINKER_FLAGS=${CMAKE_MODULE_LINKER_FLAGS}
+  -DCMAKE_MODULE_LINKER_FLAGS_INIT=${CMAKE_MODULE_LINKER_FLAGS_INIT}
   -DCMAKE_SHARED_LINKER_FLAGS=${CMAKE_SHARED_LINKER_FLAGS}
+  -DCMAKE_SHARED_LINKER_FLAGS_INIT=${CMAKE_SHARED_LINKER_FLAGS_INIT}
+  -DCMAKE_SYSROOT=${CMAKE_SYSROOT}
   -DQSYM_BACKEND=${QSYM_BACKEND}
   -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
   -DZ3_TRUST_SYSTEM_VERSION=${Z3_TRUST_SYSTEM_VERSION})
@@ -51,8 +60,8 @@ if (${TARGET_32BIT})
     SOURCE_DIR ${CMAKE_SOURCE_DIR}/runtime
     CMAKE_ARGS
     ${SYM_RUNTIME_BUILD_ARGS}
-    -DCMAKE_C_FLAGS="-m32"
-    -DCMAKE_CXX_FLAGS="-m32"
+    -DCMAKE_C_FLAGS="${CMAKE_C_FLAGS} -m32"
+    -DCMAKE_CXX_FLAGS="${CMAKE_CXX_FLAGS} -m32"
     -DZ3_DIR=${Z3_32BIT_DIR}
     -DLLVM_DIR=${LLVM_32BIT_DIR}
     INSTALL_COMMAND ""