Link against libpng and libm

Tested against Ubuntu 18.04.

CMakeLists.txt

@@ -29,6 +29,14 @@ find_package(SDL2 2.0.1)
 find_package(SDL2_mixer 2.0.0)
 find_package(SDL2_net 2.0.0)
 
+# Check for libpng.
+find_package(PNG)
+if(PNG_FOUND)
+    set(HAVE_LIBPNG TRUE)
+endif()
+
+find_package(m)
+
 include(CheckFunctionExists)
 check_function_exists(strcasecmp HAVE_DECL_STRCASECMP)
 check_function_exists(strncasecmp HAVE_DECL_STRNCASECMP)

config.h.cin

@@ -4,5 +4,6 @@
 #cmakedefine PACKAGE_STRING "@PACKAGE_STRING@"
 #cmakedefine PROGRAM_PREFIX "@PROGRAM_PREFIX@"
 
+#cmakedefine01 HAVE_LIBPNG
 #cmakedefine01 HAVE_DECL_STRCASECMP
 #cmakedefine01 HAVE_DECL_STRNCASECMP

src/CMakeLists.txt

@@ -106,6 +106,9 @@ set(SOURCE_FILES ${COMMON_SOURCE_FILES} ${GAME_SOURCE_FILES})
 set(SOURCE_FILES_WITH_DEH ${SOURCE_FILES} ${DEHACKED_SOURCE_FILES})
 
 set(EXTRA_LIBS textscreen pcsound opl SDL2::SDL2main SDL2::SDL2 SDL2::mixer SDL2::net)
+if(PNG_FOUND)
+    list(APPEND EXTRA_LIBS PNG::PNG)
+endif()
 
 if(WIN32)
     add_executable("${PROGRAM_PREFIX}doom" WIN32 ${SOURCE_FILES_WITH_DEH} "${CMAKE_CURRENT_BINARY_DIR}/resource.rc")

textscreen/CMakeLists.txt

@@ -25,4 +25,4 @@ add_library(textscreen
 target_include_directories(textscreen
                            INTERFACE "."
                            PRIVATE "../src/")
-target_link_libraries(textscreen SDL2::SDL2)
+target_link_libraries(textscreen m SDL2::SDL2)