Add libgif-dev to Dockerfile and check for gif_lib.h in configure

Author: sh1r4s3

configure.ac

@@ -129,7 +129,7 @@ AX_CHECK_BASH_COMPLETION
 AX_CHECK_ZSH_COMPLETION
 
 # Checks for header files.
-AC_CHECK_HEADERS([fcntl.h float.h inttypes.h limits.h locale.h netinet/in.h paths.h stddef.h stdint.h stdlib.h string.h sys/param.h sys/socket.h sys/time.h unistd.h], , [AC_MSG_FAILURE([cannot find the $ac_header header, which i3lock requires])])
+AC_CHECK_HEADERS([fcntl.h float.h inttypes.h limits.h locale.h netinet/in.h paths.h stddef.h stdint.h stdlib.h string.h sys/param.h sys/socket.h sys/time.h unistd.h gif_lib.h], , [AC_MSG_FAILURE([cannot find the $ac_header header, which i3lock requires])])
 
 AC_CONFIG_FILES([Makefile])
 

travis/Dockerfile

@@ -19,7 +19,7 @@ RUN apt-get update && \
     build-essential clang git autoconf automake libxcb-randr0-dev pkg-config libpam0g-dev \
     libcairo2-dev libxcb1-dev libxcb-dpms0-dev libxcb-image0-dev libxcb-util0-dev \
     libxcb-xrm-dev libev-dev libxcb-xinerama0-dev libxcb-xkb-dev libxkbcommon-dev \
-    libxkbcommon-x11-dev clang-format-9 && \
+    libxkbcommon-x11-dev clang-format-9 libgif-dev && \
     rm -rf /var/lib/apt/lists/*
 
 WORKDIR /usr/src