cc-wrapper: Fix gnatmake wrapper

Switches that gnatmake needs to pass to gcc must be given as
"-cargs <gcc_switches>" after at least the files to compile (see the
gnatmake docs for all the subtleties).  This commit makes that happen,
in a way that triggers rebuilds only for things that actually depend on
GNAT, and not the other compilers contained in GCC.

pkgs/build-support/cc-wrapper/add-gnat-extra-flags.sh

@@ -0,0 +1,23 @@
+# See add-flags.sh in cc-wrapper for comments.
+var_templates_list=(
+    NIX_GNATMAKE_CARGS
+)
+
+accumulateRoles
+
+for var in "${var_templates_list[@]}"; do
+    mangleVarList "$var" ${role_suffixes[@]+"${role_suffixes[@]}"}
+done
+
+# `-B@out@/bin' forces cc to use wrapped as instead of the system one.
+NIX_GNATMAKE_CARGS_@suffixSalt@="$NIX_GNATMAKE_CARGS_@suffixSalt@ -B@out@/bin/"
+
+# Only add darwin min version flag if a default darwin min version is set,
+# which is a signal that we're targetting darwin.
+if [ "@darwinMinVersion@" ]; then
+    mangleVarSingle @darwinMinVersionVariable@ ${role_suffixes[@]+"${role_suffixes[@]}"}
+
+    NIX_GNATMAKE_CARGS_@suffixSalt@="-m@darwinPlatformForCC@-version-min=${@darwinMinVersionVariable@_@suffixSalt@:-@darwinMinVersion@} $NIX_GNATMAKE_CARGS_@suffixSalt@"
+fi
+
+export NIX_GNAT_WRAPPER_EXTRA_FLAGS_SET_@suffixSalt@=1

pkgs/build-support/cc-wrapper/default.nix

@@ -519,6 +519,10 @@ stdenv.mkDerivation {
       substituteAll ${../wrapper-common/utils.bash} $out/nix-support/utils.bash
     ''
 
+    + optionalString cc.langAda or false ''
+      substituteAll ${./add-gnat-extra-flags.sh} $out/nix-support/add-gnat-extra-flags.sh
+    ''
+
     ##
     ## General Clang support
     ## Needs to go after ^ because the for loop eats \n and makes this file an invalid script

pkgs/build-support/cc-wrapper/gnat-wrapper.sh

@@ -29,6 +29,9 @@ if [ -z "${NIX_CC_WRAPPER_FLAGS_SET_@suffixSalt@:-}" ]; then
     source @out@/nix-support/add-flags.sh
 fi
 
+if [ -z "${NIX_GNAT_WRAPPER_EXTRA_FLAGS_SET_@suffixSalt@:-}" ]; then
+    source @out@/nix-support/add-gnat-extra-flags.sh
+fi
 
 # Parse command line options and set several variables.
 # For instance, figure out if linker flags should be passed.
@@ -126,7 +129,7 @@ fi
 
 if [ "$(basename $0)x" = "gnatmakex" ]; then
     extraBefore=("--GNATBIND=@out@/bin/gnatbind" "--GNATLINK=@out@/bin/gnatlink")
-    extraAfter=($NIX_GNATFLAGS_COMPILE_@suffixSalt@)
+    extraAfter=($NIX_GNATFLAGS_COMPILE_@suffixSalt@ -cargs $NIX_GNATMAKE_CARGS_@suffixSalt@)
 fi
 
 if [ "$(basename $0)x" = "gnatbindx" ]; then