Enable MathJax in Doxygen.in

Note that HTTPS must be used against the MathJax CDN when hosted on `eigen.tuxfamily.org` (which uses HTTPS) in order to avoid `Mixed Content`-errors from browsers. Using HTTPS for MathJax also works if the Eigen docs are hosted on plain HTTP.

doc/CMakeLists.txt

diff --git a/doc/CMakeLists.txt b/doc/CMakeLists.txt
index 50ae9ea..0f9ef23 100644
--- a/doc/CMakeLists.txt
+++ b/doc/CMakeLists.txt
@@ -14,7 +14,7 @@
 check_cxx_compiler_flag("-std=c++11" EIGEN_COMPILER_SUPPORT_CPP11)
 
 option(EIGEN_INTERNAL_DOCUMENTATION "Build internal documentation" OFF)
-
+option(EIGEN_DOC_USE_MATHJAX "Use MathJax for rendering math in HTML docs" ON)
 
 # Set some Doxygen flags
 set(EIGEN_DOXY_PROJECT_NAME             "Eigen")
@@ -22,12 +22,19 @@
 set(EIGEN_DOXY_INPUT                    "\"${Eigen_SOURCE_DIR}/Eigen\" \"${Eigen_SOURCE_DIR}/doc\"")
 set(EIGEN_DOXY_HTML_COLORSTYLE_HUE      "220")
 set(EIGEN_DOXY_TAGFILES                 "")
+
 if(EIGEN_INTERNAL_DOCUMENTATION)
   set(EIGEN_DOXY_INTERNAL                 "YES")
 else()
   set(EIGEN_DOXY_INTERNAL                 "NO")
 endif()
 
+if (EIGEN_DOC_USE_MATHJAX)
+  set(EIGEN_DOXY_USE_MATHJAX "YES")
+else ()
+  set(EIGEN_DOXY_USE_MATHJAX "NO")
+endif()
+
 configure_file(
   ${CMAKE_CURRENT_SOURCE_DIR}/Doxyfile.in
   ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile

doc/Doxyfile.in

diff --git a/doc/Doxyfile.in b/doc/Doxyfile.in
index 72120f1..ddfd7e8 100644
--- a/doc/Doxyfile.in
+++ b/doc/Doxyfile.in
@@ -1246,7 +1246,7 @@
 # output. When enabled you may also need to install MathJax separately and
 # configure the path to it using the MATHJAX_RELPATH option.
 
-USE_MATHJAX            = NO
+USE_MATHJAX            = @EIGEN_DOXY_USE_MATHJAX@
 
 # When MathJax is enabled you need to specify the location relative to the
 # HTML output directory using the MATHJAX_RELPATH option. The destination
@@ -1258,12 +1258,12 @@
 # However, it is strongly recommended to install a local
 # copy of MathJax from http://www.mathjax.org before deployment.
 
-MATHJAX_RELPATH        = http://cdn.mathjax.org/mathjax/latest
+MATHJAX_RELPATH        = https://cdn.mathjax.org/mathjax/latest
 
 # The MATHJAX_EXTENSIONS tag can be used to specify one or MathJax extension
 # names that should be enabled during MathJax rendering.
 
-MATHJAX_EXTENSIONS     =
+MATHJAX_EXTENSIONS     = TeX/AMSmath TeX/AMSsymbols
 
 # When the SEARCHENGINE tag is enabled doxygen will generate a search box
 # for the HTML output. The underlying search engine uses javascript