Move uncritical files into etc/ (#350)

This is mainly just to reduce the amount of files in the project root.
The criteria of what gets into `etc/` are the following:

* The file should not be necessary for building nsxiv. This excludes the
  `icon/*` stuff since that's needed by `window.c`.
* The file shouldn't have any valid reason to stay in the project root.
  This excludes things like `README.md`, `.gitignore` etc.

Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/350
Reviewed-by: explosion-mental <explosion-mental@noreply.codeberg.org>

etc/woodpecker/CFLAGS

@@ -0,0 +1,13 @@
+# vanilla flags
+-std=c99 -Wall -pedantic
+# optimizations: enables extra warnings and deeper analysis thus catches more errors/warnings
+-O3 -flto
+# treat warnings as errors
+-Werror
+# extra flags
+-Wextra -Wshadow -Wvla -Wpointer-arith
+-Wundef -Wstrict-overflow=4 -Wwrite-strings -Wunreachable-code
+-Wbad-function-cast -Wdeclaration-after-statement
+-Wmissing-prototypes -Wstrict-prototypes
+# silence
+-Wno-unused-parameter -Wno-missing-field-initializers