gnu: aspell: Fix FTBFS with GCC7.

* gnu/packages/patches/aspell-gcc-compat.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/aspell.scm (aspell)[source](patches): Use it.

gnu/local.mk

@@ -643,6 +643,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/apr-skip-getservbyname-test.patch	\
   %D%/packages/patches/aria2-CVE-2019-3500.patch		\
   %D%/packages/patches/aspell-default-dict-dir.patch		\
+  %D%/packages/patches/aspell-gcc-compat.patch			\
   %D%/packages/patches/ath9k-htc-firmware-binutils.patch	\
   %D%/packages/patches/ath9k-htc-firmware-gcc.patch		\
   %D%/packages/patches/ath9k-htc-firmware-objcopy.patch		\

gnu/packages/aspell.scm

@@ -45,7 +45,8 @@
       (sha256
        (base32
         "1qgn5psfyhbrnap275xjfrzppf5a83fb67gpql0kfqv37al869gm"))
-      (patches (search-patches "aspell-default-dict-dir.patch"))))
+      (patches (search-patches "aspell-default-dict-dir.patch"
+                               "aspell-gcc-compat.patch"))))
     (build-system gnu-build-system)
     (arguments
      `(#:phases

gnu/packages/patches/aspell-gcc-compat.patch

@@ -0,0 +1,31 @@
+Fix GCC7 warnings.
+
+Taken from upstream:
+https://git.savannah.gnu.org/cgit/aspell.git/commit/?id=8089fa02122fed0a6394eba14bbedcb1d18e2384
+
+diff --git a/modules/filter/tex.cpp b/modules/filter/tex.cpp
+index a979539..19ab63c 100644
+--- a/modules/filter/tex.cpp
++++ b/modules/filter/tex.cpp
+@@ -174,7 +174,7 @@ namespace {
+ 
+     if (c == '{') {
+ 
+-      if (top.in_what == Parm || top.in_what == Opt || top.do_check == '\0')
++      if (top.in_what == Parm || top.in_what == Opt || *top.do_check == '\0')
+ 	push_command(Parm);
+ 
+       top.in_what = Parm;
+diff --git a/prog/check_funs.cpp b/prog/check_funs.cpp
+index db54f3d..89ee09d 100644
+--- a/prog/check_funs.cpp
++++ b/prog/check_funs.cpp
+@@ -647,7 +647,7 @@ static void print_truncate(FILE * out, const char * word, int width) {
+     }
+   }
+   if (i == width-1) {
+-    if (word == '\0')
++    if (*word == '\0')
+       put(out,' ');
+     else if (word[len] == '\0')
+       put(out, word, len);