projects.mk + defs.mk: Make some variables more universally usable

- Remove temporary variable PRJS_DIR
- Add defaults for BUILD_LIBS_PREFIX and BUILD_TOOLS_PREFIX
- Move definitions of varables needed early on from defs.mk
  into projects.mk, such as PYTHON and PYTHON_VERSION

Signed-off-by: Jan Lindemann <jan@janware.com>

make/defs.mk

@@ -11,6 +11,8 @@ empty                       :=
 space                       := $(empty) $(empty)
 comma                       := ,
 
+# ----- executables
+
 include $(MODDIR)/make/projects.mk
 
 ifeq ($(MAKE_BENCHMARK),true)
@@ -19,10 +21,6 @@ else
   export SHELL              := /bin/bash
 endif
 
-# ----- executables
-ifndef WHICH
-  WHICH                     := $(firstword $(wildcard /usr/bin/which) $(shell which which))
-endif
 ifndef GREP
   GREP                      := $(shell $(WHICH) grep)
 endif
@@ -157,6 +155,17 @@ else ifneq ($(wildcard $(TOPDIR)/CVS),)
   VCS                       ?= cvs
 endif
 
+ifndef BUILD_TOOLS_PREFIX
+  ifdef BUILD_LIBS_PREFIX
+    BUILD_TOOLS_PREFIX       = $(BUILD_LIBS_PREFIX)
+  endif
+endif
+ifdef BUILD_LIBS_PREFIX
+  ifndef BUILD_TOOLS_PREFIX
+    BUILD_LIBS_PREFIX        = $(BUILD_TOOLS_PREFIX)
+  endif
+endif
+
 # ----- user and organization
 
 ifndef BUILD_USER