properly test for (custom) prerequisites

Author: mcorino

rakelib/lib/config/linux.rb

@@ -61,7 +61,7 @@ def check_tool_pkgs
             if get_config('with-wxwin') && get_cfg_string('wxwin').empty?
               pkg_deps << 'patchelf' unless system('command -v patchelf>/dev/null')
               pkg_deps << 'make' unless system('command -v make>/dev/null')
-              pkg_deps << 'git' unless system('command -v git>/dev/null')
+              pkg_deps << 'git' unless system("command -v #{get_cfg_string('git')}>/dev/null")
             end
             pkg_deps
           end

rakelib/lib/config/unixish.rb

@@ -58,8 +58,8 @@ def do_link(pkg)
 
       def check_tool_pkgs
         pkg_deps = super
-        pkg_deps << 'doxygen' unless system('command -v doxygen>/dev/null')
-        pkg_deps << 'swig' unless system('command -v swig>/dev/null')
+        pkg_deps << 'doxygen' unless system("command -v #{get_cfg_string('doxygen')}>/dev/null")
+        pkg_deps << 'swig' unless system("command -v #{get_cfg_string('swig')}>/dev/null")
         pkg_deps
       end