update build_tests_spec to invoke rscons in a subprocess for each test

build_tests/two_sources/Rsconsfile

@@ -0,0 +1,4 @@
+Rscons::Environment.new(echo: :command) do |env|
+  env.Object("one.o", "one.c", 'CPPFLAGS' => ['-DONE'])
+  env.Program('two_sources.exe', ['one.o', 'two.c'])
+end

build_tests/two_sources/assuffix.rb

@@ -0,0 +1,7 @@
+Rscons::Environment.new do |env|
+  env["ASSUFFIX"] = %w[.ssss .sss]
+  env["CFLAGS"] += %w[-S]
+  env.Object("one.ssss", "one.c", "CPPFLAGS" => ["-DONE"])
+  env.Object("two.sss", "two.c")
+  env.Program("two_sources.exe", %w[one.ssss two.sss], "ASFLAGS" => env["ASFLAGS"] + %w[-x assembler])
+end

build_tests/two_sources/libsuffix.rb

@@ -0,0 +1,6 @@
+Rscons::Environment.new() do |env|
+  env["LIBSUFFIX"] = %w[.aaaa .aaa]
+  env.Library("one.aaaa", "one.c", "CPPFLAGS" => ["-DONE"])
+  env.Library("two.aaa", "two.c")
+  env.Program("two_sources.exe", %w[one.aaaa two.aaa])
+end

build_tests/two_sources/objsuffix.rb

@@ -0,0 +1,6 @@
+Rscons::Environment.new do |env|
+  env["OBJSUFFIX"] = %w[.oooo .ooo]
+  env.Object("one.oooo", "one.c", "CPPFLAGS" => ["-DONE"])
+  env.Object("two.ooo", "two.c")
+  env.Program("two_sources.exe", %w[one.oooo two.ooo])
+end