Allowing Doxygen to built html and man doc in specified directory

following recommendation from @albert-github

Author: julielangou

Makefile

@@ -120,7 +120,7 @@ lapacke_example: lapackelib lapacklib blaslib
 .PHONY: html
 html:
 	@echo "LAPACK HTML PAGES GENERATION with Doxygen"
-	doxygen DOCS/Doxyfile
+	( cat DOCS/Doxyfile ; echo "OUTPUT_DIRECTORY=$(DOCSDIR)" ) | doxygen -
 	@echo "=================="
 	@echo "LAPACK HTML PAGES GENERATED in DOCS/explore-html"
 	@echo "Usage: open DOCS/explore-html/index.html"
@@ -130,9 +130,10 @@ html:
 .PHONY: man
 man:
 	@echo "LAPACK MAN PAGES GENERATION with Doxygen"
-	doxygen DOCS/Doxyfile_man
+	@echo "OUTPUT_DIRECTORY=$(DOCSDIR)"
+	( cat DOCS/Doxyfile_man ; echo "OUTPUT_DIRECTORY=$(DOCSDIR)" ) | doxygen -
 	@echo "=================="
-	@echo "LAPACK MAN PAGES GENERATED in DOCS/MAN"
+	@echo "LAPACK MAN PAGES GENERATED in DOCS/man"
 	@echo "Set your MANPATH env variable accordingly"
 	@echo "Usage: man dgetrf.f"
 	@echo "=================="

make.inc.example

@@ -81,3 +81,8 @@ CBLASLIB     = $(TOPSRCDIR)/libcblas.a
 LAPACKLIB    = $(TOPSRCDIR)/liblapack.a
 TMGLIB       = $(TOPSRCDIR)/libtmglib.a
 LAPACKELIB   = $(TOPSRCDIR)/liblapacke.a
+
+#  DOCUMENTATION DIRECTORY
+# If you generate html pages (make html), documentation will be placed in $(DOCSDIR)/explore-html
+# If you generate man pages (make man), documentation will be placed in $(DOCSDIR)/man
+DOCSDIR       = $(TOPSRCDIR)/DOCS