m4: make explicit what boost library we are not finding

Change-Id: Icd4e926a6e73ea1147419a9190d7888b6ac3e4c7
Reviewed-on: https://gerrit.libreoffice.org/28312Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Jenkins <ci@libreoffice.org>

m4/ax_boost_date_time.m4

@@ -100,7 +100,7 @@ AC_DEFUN([AX_BOOST_DATE_TIME],
 
             fi
             if test "x$ax_lib" = "x"; then
-                AC_MSG_ERROR(Could not find a version of the library!)
+                AC_MSG_ERROR(Could not find a version of the boost date_time library!)
             fi
 			if test "x$link_date_time" != "xyes"; then
 				AC_MSG_ERROR(Could not link against $ax_lib !)

m4/ax_boost_filesystem.m4

@@ -104,7 +104,7 @@ AC_DEFUN([AX_BOOST_FILESYSTEM],
 
             fi
             if test "x$ax_lib" = "x"; then
-                AC_MSG_ERROR(Could not find a version of the library!)
+                AC_MSG_ERROR(Could not find a version of the boost filesystem library!)
             fi
 			if test "x$link_filesystem" != "xyes"; then
 				AC_MSG_ERROR(Could not link against $ax_lib !)

m4/ax_boost_iostreams.m4

@@ -103,7 +103,7 @@ AC_DEFUN([AX_BOOST_IOSTREAMS],
 
             fi
             if test "x$ax_lib" = "x"; then
-                AC_MSG_ERROR(Could not find a version of the library!)
+                AC_MSG_ERROR(Could not find a version of the boost iostreams library!)
             fi
 			if test "x$link_iostreams" != "xyes"; then
 				AC_MSG_ERROR(Could not link against $ax_lib !)

m4/ax_boost_system.m4

@@ -108,7 +108,7 @@ AC_DEFUN([AX_BOOST_SYSTEM],
 
             fi
             if test "x$ax_lib" = "x"; then
-                AC_MSG_ERROR(Could not find a version of the library!)
+                AC_MSG_ERROR(Could not find a version of the boost system library!)
             fi
 			if test "x$link_system" = "xno"; then
 				AC_MSG_ERROR(Could not link against $ax_lib !)