Allow empty free optional feature valus in CLI.

Features that are narked as 'free' and 'optional' will now be
ignored when the value specified on the command line is
empty. Hence once can specify `cxxflags=` on the command
line without errors. All current "flags" features are now optional.

fixes #5

doc/src/history.adoc

@@ -11,6 +11,10 @@
 * *New:* Include pch header automatically and on-demand on gcc and msvc toolset to
   mirror clang functionality.
   -- _Nikita Kniazev_
+* *New:* Features that are narked as 'free' and 'optional' will now be ignored when
+  the value specified on the command line is empty. Hence once can specify
+  `cxxflags=` on the command line without errors.
+  -- _René Ferdinand Rivera Morell_
 * Preserve `bootstrap.sh` invoke arguments to forward to the `build.sh` script.
   -- _tkoecker_
 * Remove use of `local` in `buils.sh` to be compatible with some, not fully

src/build-system.jam

@@ -771,14 +771,22 @@ local rule should-clean-project ( project )
     # on the command line) have been loaded.
     if $(properties)
     {
-        expanded += [ build-request.convert-command-line-elements $(properties) ] ;
-        expanded = [ build-request.expand-no-defaults $(expanded) ] ;
-        local xexpanded ;
-        for local e in $(expanded)
+        local cli_properties = [ build-request.convert-command-line-elements $(properties) ] ;
+        if $(cli_properties)
         {
-            xexpanded += [ property-set.create [ feature.split $(e) ] ] ;
+            expanded += $(cli_properties) ;
+            expanded = [ build-request.expand-no-defaults $(expanded) ] ;
+            local xexpanded ;
+            for local e in $(expanded)
+            {
+                xexpanded += [ property-set.create [ feature.split $(e) ] ] ;
+            }
+            expanded = $(xexpanded) ;
+        }
+        else
+        {
+            expanded = [ property-set.empty ] ;
         }
-        expanded = $(xexpanded) ;
     }
     else
     {

src/build/build-request.jam

@@ -258,7 +258,22 @@ local rule convert-command-line-element ( e )
             values = $(values:J=,) ;
             values = $(values) $(parts[2-]) ;
             values = $(values:J=/) ;
-            lresult = <$(feature)>$(values) ;
+            lresult = ;
+            # Optional free features will ignore empty value arguments.
+            if optional in [ feature.attributes <$(feature)> ]
+            {
+                for local v in $(values)
+                {
+                    if $(v)
+                    {
+                        lresult += <$(feature)>$(v) ;
+                    }
+                }
+            }
+            else
+            {
+                lresult = <$(feature)>$(values) ;
+            }
             parts = ;
         }
 
@@ -272,13 +287,16 @@ local rule convert-command-line-element ( e )
             }
         }
 
-        if ! $(result)
-        {
-            result = $(lresult) ;
-        }
-        else
+        if $(lresult)
         {
-            result = $(result)/$(lresult) ;
+            if ! $(result)
+            {
+                result = $(lresult) ;
+            }
+            else
+            {
+                result = $(result)/$(lresult) ;
+            }
         }
 
         parts = $(parts[2-]) ;

src/tools/features/archiveflags-feature.jam

@@ -15,4 +15,4 @@ when creating static libraries.
 
 feature.feature archiveflags
     :
-    : free ;
+    : free optional ;

src/tools/features/asmflags-feature.jam

@@ -14,4 +14,4 @@ The value of this feature is passed without modification to the assembler.
 
 feature.feature asmflags
     :
-    : free ;
+    : free optional ;

src/tools/features/cflags-feature.jam

@@ -18,4 +18,4 @@ higher-level feature in B2.
 
 feature.feature cflags
     :
-    : free ;
+    : free optional ;

src/tools/features/compileflags-feature.jam

@@ -16,4 +16,4 @@ language for the tools.
 
 feature.feature compileflags
     :
-    : free ;
+    : free optional ;

src/tools/features/cxxflags-feature.jam

@@ -14,4 +14,4 @@ See <<bbv2.builtin.features.cflags,`<cflags>`>>.
 
 feature.feature cxxflags
     :
-    : free ;
+    : free optional ;

src/tools/features/fflags-feature.jam

@@ -15,4 +15,4 @@ compiling Fortran sources.
 
 feature.feature fflags
     :
-    : free ;
+    : free optional ;

src/tools/features/flags-feature.jam

@@ -16,4 +16,4 @@ build the target.
 
 feature.feature flags
     :
-    : free ;
+    : free optional ;

src/tools/features/linkflags-feature.jam

@@ -14,4 +14,4 @@ See <<bbv2.builtin.features.cflags,`<cflags>`>>.
 
 feature.feature linkflags
     :
-    : free ;
+    : free optional ;

src/tools/features/objcflags-feature.jam

@@ -16,7 +16,7 @@ compiling Objective C sources.
 
 feature.feature mflags
     :
-    : free ;
+    : free optional ;
 
 #| tag::doc[]
 
@@ -29,4 +29,4 @@ compiling Objective {CPP} sources.
 
 feature.feature mmflags
     :
-    : free ;
+    : free optional ;