Prepare for updated toolchains (#211)

* provide -rpath-link by default

This fixes the link with depended upon libraries, which are not found
when this flag is lacking and the libraries are not explicitely
provided.
This is basically the equivalent of `-L` for libraries that are listed
in the `DT_NEEDED` section of a library boing linked with.
This is fixing some issues with (at least) curl & gcrypt, but I
introduced this before trying to build other dependencies so there are
likely more.
If I'm being honest I don't understand why this wasn't required before,
I'm guessing some change in libtool, but I wasn't able to identify
something precisely.

* allow environment to override compiler & cmake toolchain

* simplify environment variables forwarding

* keep rubocop happy

* use symbols instead of strings

* fix env forwarding

lib/omnibus/builder.rb

@@ -241,9 +241,13 @@ def cmake(*args)
       cmake_cmd << "-DCMAKE_INSTALL_LIBDIR=#{libdir}" if libdir && libdir != ""
       rpath = options.delete(:rpath) || "#{prefix}/#{libdir}"
       cmake_cmd << "-DCMAKE_INSTALL_RPATH=#{rpath}" if rpath && rpath != ""
+
+      if ENV['DD_CMAKE_TOOLCHAIN']
+        cmake_cmd << "--toolchain #{ENV['DD_CMAKE_TOOLCHAIN']}"
+      end
+
       cmake_cmd.concat args
       cmake_cmd = cmake_cmd.join(" ").strip
-
       command(cmake_cmd, options)
 
       make("-j #{workers}", options)

lib/omnibus/software.rb

@@ -785,11 +785,27 @@ def with_standard_compiler_flags(env = {}, opts = {})
             "CFLAGS" => "-I#{install_dir}/embedded/include #{arch_flag} -O2 -fno-lto #{opt_flag}",
           }
         else
-          {
+          flags = {
             # -z origin makes the rpath interpret the $ORIGIN variable as the binary path
-            "LDFLAGS" => "-Wl,-rpath,#{install_dir}/embedded/lib,-z,origin -L#{install_dir}/embedded/lib",
+            "LDFLAGS" => "-Wl,-rpath,#{install_dir}/embedded/lib,-z,origin -L#{install_dir}/embedded/lib -Wl,-rpath-link=#{install_dir}/embedded/lib",
             "CFLAGS" => "-I#{install_dir}/embedded/include -O2",
           }
+          # List of environment variables to forward and potentially map to an alternate name
+          # If the value is nil, the key will simply be forwarded from ENV to flags
+          to_forward = {
+            DD_CC: "CC",
+            DD_CXX: "CXX",
+            DD_CMAKE_TOOLCHAIN: nil,
+          }
+          to_forward.each do |orig, forwarded|
+            if ENV[orig.to_s]
+              flags[forwarded] = ENV[orig.to_s] if forwarded
+              # Forward the toolchain env since some invoke tasks
+              # rely on it down the line
+              flags[orig.to_s] = ENV[orig.to_s]
+            end
+          end
+          flags
         end
 
       # merge LD_RUN_PATH into the environment.  most unix distros will fall