Mechanism to override AC_CHECK_FUNCS in Autoconf

For R packages using Autoconf, we'll sometimes need to be able to
force the result of AC_CHECK_FUNCS.

For example, for the `uuid` package Autoconf sets `HAVE_GETRANDOM`.
Despite compiling OK, using `getrandom()` at the time of writing
in fact fails for R packages compiled using Emscripten, due to a
missing symbol. Autoconf missed this because we compile R packages
with the `SIDE_MODULE=1` flag, and with this flag Emscripten does
not complain about the missing symbols.

So, using this mechanism we can force Autoconf to return "no" when
checking for `getrandom()`, fixing the issue with `uuid`.

inst/configure

@@ -9,5 +9,8 @@
 # Additional specific R package fixes
 export STRINGI_CPPFLAGS=-DU_HAVE_MMAP=0
 
+# Override specific Autoconf results for Emscripten environment
+AC_FUNC_OVERRIDES='ac_cv_func_getrandom=no'
+
 # sh is needed for scripts without shebang
-emconfigure sh -c "./configure.orig --build=$BUILD_PLATFORM --host=wasm32-unknown-emscripten"
+emconfigure sh -c "./configure.orig --build=$BUILD_PLATFORM --host=wasm32-unknown-emscripten $AC_FUNC_OVERRIDES"