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4.5.1 'VPATH': Search Path for All Prerequisites
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The value of the 'make' variable 'VPATH' specifies a list of directories
that 'make' should search.  Most often, the directories are expected to
contain prerequisite files that are not in the current directory;
however, 'make' uses 'VPATH' as a search list for both prerequisites and
targets of rules.

   Thus, if a file that is listed as a target or prerequisite does not
exist in the current directory, 'make' searches the directories listed
in 'VPATH' for a file with that name.  If a file is found in one of
them, that file may become the prerequisite (see below).  Rules may then
specify the names of files in the prerequisite list as if they all
existed in the current directory.  *Note Writing Recipes with Directory
Search: Recipes/Search.

   In the 'VPATH' variable, directory names are separated by colons or
blanks.  The order in which directories are listed is the order followed
by 'make' in its search.  (On MS-DOS and MS-Windows, semi-colons are
used as separators of directory names in 'VPATH', since the colon can be
used in the pathname itself, after the drive letter.)

   For example,

     VPATH = src:../headers

specifies a path containing two directories, 'src' and '../headers',
which 'make' searches in that order.

   With this value of 'VPATH', the following rule,

     foo.o : foo.c

is interpreted as if it were written like this:

     foo.o : src/foo.c

assuming the file 'foo.c' does not exist in the current directory but is
found in the directory 'src'.