Install the documentation under $(datadir), not $(prefix).

Makefile.msvc

@@ -41,10 +41,11 @@ srcdir = .
 # Directories used by "make install":
 prefix = c:\usr
 exec_prefix = $(prefix)
+datadir = $(prefix)\share
 bindir = $(exec_prefix)\bin
-mandir = $(prefix)\man
+mandir = $(datadir)\man
 man1dir = $(mandir)\man1
-docdir = $(prefix)\doc\gperf
+docdir = $(datadir)\doc\gperf
 
 # Programs used by "make":
 
@@ -155,10 +156,11 @@ install : all force
 	-mkdir $(exec_prefix)
 	-mkdir $(bindir)
 	$(INSTALL_PROGRAM) gperf.exe $(bindir)\gperf.exe
+	-mkdir $(datadir)
 	-mkdir $(mandir)
 	-mkdir $(man1dir)
 	$(INSTALL_DATA) doc\gperf.1 $(man1dir)\gperf.1
-	-mkdir $(prefix)\doc
+	-mkdir $(datadir)\doc
 	-mkdir $(docdir)
 	$(INSTALL_DATA) doc\gperf.html $(docdir)\gperf.html
 
@@ -166,9 +168,10 @@ installdirs : force
 	-mkdir $(prefix)
 	-mkdir $(exec_prefix)
 	-mkdir $(bindir)
+	-mkdir $(datadir)
 	-mkdir $(mandir)
 	-mkdir $(man1dir)
-	-mkdir $(prefix)\doc
+	-mkdir $(datadir)\doc
 	-mkdir $(docdir)
 
 uninstall : force