check-doc-syntax: Don't hardcode path to awk

Instead of expecting awk in /usr/bin, this commit changes the code to call awk
through a shell so that $PATH is searched.

Since this awk script shouldn't really be called manually, this is done by
removing the shebang from the awk script, marking it non-executable and fixing
up the caller.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67674
Reviewed-by: Bryce Harrington <b.harrington@samsung.com>
Signed-off-by: Uli Schlachter <psychon@znc.in>

src/check-doc-syntax.awk

@@ -1,5 +1,3 @@
-#!/usr/bin/awk -f
-
 BEGIN {
     name_found = 1
     SECTION_DOC = 0

src/check-doc-syntax.sh

@@ -72,7 +72,7 @@ fi >&2
 
 # Only run the syntax checker on the source files (not doc/)
 if test -e ./check-doc-syntax.awk; then
-    if echo $FILES | xargs ./check-doc-syntax.awk ; then
+    if echo $FILES | xargs awk -f ./check-doc-syntax.awk ; then
 	    :
     else
 	    stat=1