Force release of free memory

This should not be needed, but i can totally reproduce that without it
top is still reporting lots of memory used by Okular when it's really
not (heaptrack reports memory freed correctly)

BUGS: 394834

OkularConfigureChecks.cmake

@@ -10,6 +10,10 @@ else (OKULAR_FORCE_DRM)
    set(_OKULAR_FORCE_DRM 0)
 endif (OKULAR_FORCE_DRM)
 
+# Check whether malloc_trim(3) is supported.
+include(CheckSymbolExists)
+check_symbol_exists(malloc_trim "malloc.h" HAVE_MALLOC_TRIM)
+
 # at the end, output the configuration
 configure_file(
    ${CMAKE_CURRENT_SOURCE_DIR}/config-okular.h.cmake

config-okular.h.cmake

@@ -4,3 +4,6 @@
 /* Defines if the purpose framework is available */
 #define PURPOSE_FOUND ${PURPOSE_FOUND}
 
+/* Defines if the purpose framework is available */
+#cmakedefine01 HAVE_MALLOC_TRIM
+

core/document.cpp

@@ -98,6 +98,10 @@
 
 #include <config-okular.h>
 
+#if HAVE_MALLOC_TRIM
+#include "malloc.h"
+#endif
+
 using namespace Okular;
 
 struct AllocatedPixmap
@@ -2724,6 +2728,13 @@ void Document::closeDocument()
 
     d->m_undoStack->clear();
     d->m_docdataMigrationNeeded = false;
+
+#if HAVE_MALLOC_TRIM
+    // trim unused memory, glibc should do this but it seems it does not
+    // this can greatly decrease the [perceived] memory consumption of okular
+    // see: https://sourceware.org/bugzilla/show_bug.cgi?id=14827
+    malloc_trim(0);
+#endif
 }
 
 void Document::addObserver( DocumentObserver * pObserver )