Update how C API headers are installed (#11478)

Currently a script is used which means that at install-time a `*.cmake`
script is run. While I thought this inherited CMake configuration
options turns out it doesn't so the installed headers don't actually
reflect the configured features at build time. This commit changes this
to install from the headers-in-the-build-directory which indeed do
reflect features.

Author: alexcrichton

crates/c-api/CMakeLists.txt

@@ -109,7 +109,7 @@ set(WASMTIME_HEADER_DST ${CMAKE_BINARY_DIR}/include)
 include(cmake/install-headers.cmake)
 
 include(GNUInstallDirs)
-install(SCRIPT ${CMAKE_CURRENT_SOURCE_DIR}/cmake/install-headers.cmake)
+install(DIRECTORY "${WASMTIME_HEADER_DST}/" TYPE INCLUDE)
 install(FILES ${WASMTIME_SHARED_FILES} ${WASMTIME_STATIC_FILES}
         DESTINATION ${CMAKE_INSTALL_LIBDIR})