Add getenv macro, use instead of $(echo $HOME) for defauls

- inspired by http://lists.rpm.org/pipermail/rpm-maint/2009-January/002332.html

lib/rpmrc.c

@@ -475,7 +475,7 @@ static void setDefaults(void)
     addMacro(NULL, "_preScriptEnvironment",NULL, prescriptenviron,RMIL_DEFAULT);
 
     addMacroDefault("_topdir",
-		"/usr/src/packages",		"%(echo $HOME)/rpmbuild");
+		"/usr/src/packages",		"%{getenv:HOME}/rpmbuild");
     addMacroDefault("_tmppath",
 		LOCALSTATEDIR "/tmp",		"%{_var}/tmp");
     addMacroDefault("_dbpath",

macros.in

@@ -229,7 +229,7 @@ package or when debugging this package.\
 %_tmppath		%{_var}/tmp
 
 #	Path to top of build area.
-%_topdir		%(echo $HOME)/rpmbuild
+%_topdir		%{getenv:HOME}/rpmbuild
 
 #	The path to the unzip executable (legacy, use %{__unzip} instead).
 %_unzipbin		%{__unzip}

rpmio/macro.c

@@ -983,6 +983,8 @@ doFoo(MacroBuf mb, int negate, const char * f, size_t fn,
             break;
 	}
 	b = be;
+    } else if (STREQ("getenv", f, fn)) {
+	b = getenv(buf);
     } else if (STREQ("S", f, fn)) {
 	for (b = buf; (c = *b) && risdigit(c);)
 	    b++;
@@ -1253,6 +1255,7 @@ expandMacro(MacroBuf mb)
 	    STREQ("uncompress", f, fn) ||
 	    STREQ("url2path", f, fn) ||
 	    STREQ("u2p", f, fn) ||
+	    STREQ("getenv", f, fn) ||
 	    STREQ("S", f, fn) ||
 	    STREQ("P", f, fn) ||
 	    STREQ("F", f, fn)) {