fix: install matplot_opengl in CMake

Author: aminya

source/matplot/CMakeLists.txt

@@ -90,6 +90,8 @@ add_library(matplot
         freestanding/plot.h
 )
 
+set(TARGETS matplot)
+
 # Target aliases
 add_library(Matplot++::matplot ALIAS matplot)
 add_library(Matplot++::matplot++ ALIAS matplot)
@@ -220,7 +222,7 @@ if (MATPLOTPP_BUILD_EXPERIMENTAL_OPENGL_BACKEND)
         # Use CPM only if not found, to avoid ODR violations
         # find_package(GLAD REQUIRE) would suffice if it worked well
         FetchContent_Declare(glad GIT_REPOSITORY https://github.com/Dav1dde/glad.git GIT_TAG df8e9e16110b305479a875399cee13daa0ccadd9)
-        FetchContent_MakeAvailable(glad)        
+        FetchContent_MakeAvailable(glad)
     else ()
         # FindGLAD does not usually create a target, so we create an interface target
         if (NOT TARGET glad)
@@ -245,16 +247,17 @@ if (MATPLOTPP_BUILD_EXPERIMENTAL_OPENGL_BACKEND)
             backend/opengl.h
             backend/opengl.cpp
             )
-    target_include_directories(matplot_opengl PUBLIC matplot)
     target_link_libraries(matplot_opengl PUBLIC matplot glad glfw ${CMAKE_DL_LIBS})
+
+    list(APPEND TARGETS matplot_opengl)
 endif()
 
 #######################################################
 ### Installer                                       ###
 #######################################################
 if (MATPLOTPP_BUILD_INSTALLER)
     # Install targets
-    install(TARGETS matplot
+    install(TARGETS ${TARGETS}
             EXPORT Matplot++Targets
 	    RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
             LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}