Use CMake's built-in timestamp formatting.

It notably honors SOURCE_DATE_EPOCH if set, making for reproducible
output.  For even better reproducibility, use UTC.  (Unlike the date
command's output, the result is already always in English.)  Extend
this approach to the book, introducing appropriately formatted
PDF_DATE and TODAY variables for its PDF metadata and title page
respectively and making make_pdf configurable.

documentation/CMakeLists.txt

@@ -37,16 +37,12 @@ if(GENERATE_DOCS)
 
   # create required variables
 
-  execute_process(COMMAND date "+%Y"
-    OUTPUT_VARIABLE YEAR
-    OUTPUT_STRIP_TRAILING_WHITESPACE
-  )
-
+  string(TIMESTAMP YEAR "%Y" UTC)
   # note: current locale is used for abbreviated month
-  execute_process(COMMAND date "+%b %d, %Y"
-    OUTPUT_VARIABLE CURRENT_DATE
-    OUTPUT_STRIP_TRAILING_WHITESPACE
-  )
+  string(TIMESTAMP CURRENT_DATE "%b %d, %Y" UTC)
+  string(TIMESTAMP PDF_DATE "D:%Y%m%d%H%M%SZ" UTC)
+  string(TIMESTAMP TODAY "%B %d, %Y" UTC)
+  string(REPLACE " 0" " " TODAY "${TODAY}")
 
   # Find "short" doxygen version if it was built from Git
   # Note: this is still needed in CMake 3.15 but later CMake versions
@@ -77,6 +73,8 @@ if(GENERATE_DOCS)
   if(0) # debug
     fl_debug_var(YEAR)
     fl_debug_var(CURRENT_DATE)
+    fl_debug_var(PDF_DATE)
+    fl_debug_var(TODAY)
     fl_debug_var(FLTK_GIT_REVISION)
     fl_debug_var(DOXYGEN_FOUND)
     fl_debug_var(DOXYGEN_EXECUTABLE)
@@ -179,7 +177,7 @@ if(FLTK_BUILD_PDF_DOCS)
     COMMENT "Converting ${DOXYFILE} to doxygen version ${DOXYGEN_VERSION_SHORT}" VERBATIM
   )
 
-  # generate LaTeX title fltk-title.tex
+  # generate LaTeX title fltk-title.tex and make_pdf script
 
   configure_file(
     ${CMAKE_CURRENT_SOURCE_DIR}/src/fltk-title.tex.in
@@ -187,6 +185,12 @@ if(FLTK_BUILD_PDF_DOCS)
     @ONLY
   )
 
+  configure_file(
+    ${CMAKE_CURRENT_SOURCE_DIR}/make_pdf.in
+    ${CMAKE_CURRENT_BINARY_DIR}/make_pdf
+    @ONLY
+  )
+
   # generate fltk.pdf
 
   add_custom_command(
@@ -196,7 +200,7 @@ if(FLTK_BUILD_PDF_DOCS)
             ${CMAKE_CURRENT_BINARY_DIR}/fltk-title.tex
             ${CMAKE_CURRENT_BINARY_DIR}/fltk-book.tex
     COMMAND ${DOXYGEN_EXECUTABLE} ${DOXYFILE}
-    COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/make_pdf
+    COMMAND ${CMAKE_CURRENT_BINARY_DIR}/make_pdf
     COMMAND cp -f latex/refman.pdf fltk.pdf
     DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/${DOXYFILE}
             ${CMAKE_CURRENT_BINARY_DIR}/fltk-title.tex

documentation/make_pdf.in

@@ -26,10 +26,15 @@
 #
 # Used in: Makefile and CMakeLists.txt
 
+run_pdflatex() {
+  pdflatex --interaction=nonstopmode \
+    "\pdfinfo{/CreationDate(@PDF_DATE@)/ModDate(@PDF_DATE@)}\input{refman.tex}"
+}
+
 ( cd latex
-  pdflatex --interaction=nonstopmode refman.tex
+  run_pdflatex
   makeindex refman.idx
-  pdflatex --interaction=nonstopmode refman.tex
+  run_pdflatex
   latex_count=5
   while egrep -s 'Rerun (LaTeX|to get cross-references right)' refman.log \
 	  && [ $latex_count -gt 0 ]

documentation/src/fltk-title.tex.in

@@ -25,7 +25,7 @@ provided this copyright and permission notice are preserved.}\\
 \vspace*{1.5cm}
 {\large Generated by Doxygen @DOXY_VERSION@}\\
 \vspace*{0.5cm}
-\today{}\\
+@TODAY@\\
 \vspace*{0.5cm}
 {\small Git revision @FLTK_GIT_REVISION@}\\
 \end{center}