Allow building WAMR without an internet connection

[alexreinking]

Is your feature request related to a problem? Please describe.
Currently, WAMR fetches simde as a dependency via FetchContent unconditionally.

Describe the solution you'd like
Upstream simde provides a find_package compatible package layout. This should be tried first. Older versions don't provide a find_package module, but a custom Find module could be written or even a simple find_path suffices.

Describe alternatives you've considered
The alternative is to maintain patches to the WAMR build system or write a complicated Dependency Provider in CMake that intercepts the FetchContent call.

Additional context
A number of package managers (including Homebrew, vcpkg, Debian, and others) disable FetchContent to ensure a consistent set of packages exists in the index.

[alexreinking]

One potential resolution could be to do this:

diff --git a/core/iwasm/libraries/simde/simde.cmake b/core/iwasm/libraries/simde/simde.cmake
--- a/core/iwasm/libraries/simde/simde.cmake
+++ b/core/iwasm/libraries/simde/simde.cmake
@@ -15,14 +15,5 @@ add_definitions (-DWASM_ENABLE_SIMDE=1)
 
 include_directories(${LIB_SIMDE_DIR} ${LIB_SIMDE_DIR}/simde)
 
-include(FetchContent)
-
-FetchContent_Declare(
-    simde
-    GIT_REPOSITORY  https://github.com/simd-everywhere/simde
-    GIT_TAG v0.8.2
-)
-
-message("-- Fetching simde ..")
-FetchContent_MakeAvailable(simde)
+find_path(simde_SOURCE_DIR simde/wasm/simd128.h)
 include_directories("${simde_SOURCE_DIR}")

Or, perhaps, fall back to FetchContent if simde is not found.