[FS#1091] libiconv-full undefined reference compile-time linking error for x86/_64 using GCC7 Musl

[LEDE Bugs]

The following task has a new comment added:

FS#1091 - libiconv-full undefined reference compile-time linking error for x86/_64 using GCC7 Musl
User who did this - Jake Staehle (staehle)

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Adding '-std=gnu89' to the LEDE package Makefile TARGET_CFLAGS results in a successful compilation.
Great answer, harrylwc! 
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=1091#comment3675