build-system: $PREFIX can now be used in YAML config file

Pmodules/libpbuild.bash

@@ -26,7 +26,6 @@ declare -a PATCH_FILES=()
 declare -a PATCH_STRIPS=()
 declare -a PATCH_STRIP_DEFAULT='1'
 declare -- configure_with='auto'
-declare -- PREFIX=''
 declare -- SRC_DIR=''
 declare -- BUILD_DIR=''
 declare -- is_subpkg='no'

Pmodules/modbuild.in

@@ -685,6 +685,7 @@ declare -x V_MAJOR=''		# first number in version string
 declare -x V_MINOR=''		# second number in version string (or empty)
 declare -x V_PATCHLVL=''	# third number in version string (or empty)
 declare -x V_RELEASE=''		# module release (or empty)
+declare -x PREFIX=''
 
 declare -A SHASUMS=()
 declare -a MODULE_DOCFILES=()