encoding.iconv: add flag for OpenBSD to find iconv include and library

[lcheylus]

Fix #23573

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Tests for iconv OK on OpenBSD current/amd64 with tcc, egcc and clang

• Tests with tcc

$ ./v -stats test vlib/encoding/iconv/                                                                                                                                                                    
---- Testing... -----
running tests in: /home/fox/dev/vlang.git/vlib/encoding/iconv/iconv_test.v                                                                                                                                                                                                        
      OK    [1/5]     0.124 ms     8 asserts | main.test_vstring_to_encoding()                                                                                                                                                                                                    
      OK    [2/5]     0.060 ms     8 asserts | main.test_encoding_to_vstring()                                                                                                                                                                                                    
      OK    [3/5]     0.033 ms     5 asserts | main.test_create_utf_string_with_bom()                                                                                                                                                                                             
      OK    [4/5]     0.027 ms     5 asserts | main.test_remove_utf_string_with_bom()
      OK    [5/5]     3.168 ms    30 asserts | main.test_read_file_encoding_write_file_encoding()
     Summary for running V tests in "iconv_test.v": 56 passed, 56 total. Elapsed time: 3 ms.
        V  source  code size:      29427 lines,     794873 bytes,   278 types,    13 modules,   131 files
generated  target  code size:      11673 lines,     447037 bytes
compilation took: 664.205 ms, compilation speed: 44304 vlines/s, cgen threads: 3

 OK     723.901 ms vlib/encoding/iconv/iconv_test.v
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• Tests with gcc (binary = egcc)

$ ./v -cc egcc -stats test vlib/encoding/iconv/
---- Testing... ----
running tests in: /home/fox/dev/vlang.git/vlib/encoding/iconv/iconv_test.v
      OK    [1/5]     0.192 ms     8 asserts | main.test_vstring_to_encoding()
      OK    [2/5]     0.061 ms     8 asserts | main.test_encoding_to_vstring()
      OK    [3/5]     0.082 ms     5 asserts | main.test_create_utf_string_with_bom()
      OK    [4/5]     0.024 ms     5 asserts | main.test_remove_utf_string_with_bom()
      OK    [5/5]     2.673 ms    30 asserts | main.test_read_file_encoding_write_file_encoding()
     Summary for running V tests in "iconv_test.v": 56 passed, 56 total. Elapsed time: 3 ms.
        V  source  code size:      29427 lines,     794873 bytes,   278 types,    13 modules,   131 files
generated  target  code size:      11659 lines,     446835 bytes
compilation took: 2658.985 ms, compilation speed: 11067 vlines/s, cgen threads: 3

 OK    2717.165 ms vlib/encoding/iconv/iconv_test.v
----

• Tests with cglang

$ ./v -cc clang -stats test vlib/encoding/iconv/
---- Testing... ----
running tests in: /home/fox/dev/vlang.git/vlib/encoding/iconv/iconv_test.v
      OK    [1/5]     0.248 ms     8 asserts | main.test_vstring_to_encoding()
      OK    [2/5]     0.059 ms     8 asserts | main.test_encoding_to_vstring()
      OK    [3/5]     0.176 ms     5 asserts | main.test_create_utf_string_with_bom()
      OK    [4/5]     0.022 ms     5 asserts | main.test_remove_utf_string_with_bom()
      OK    [5/5]     4.773 ms    30 asserts | main.test_read_file_encoding_write_file_encoding()
     Summary for running V tests in "iconv_test.v": 56 passed, 56 total. Elapsed time: 5 ms.
        V  source  code size:      29427 lines,     794873 bytes,   278 types,    13 modules,   131 files
generated  target  code size:      11659 lines,     446835 bytes
compilation took: 1972.469 ms, compilation speed: 14918 vlines/s, cgen threads: 3

 OK    2057.521 ms vlib/encoding/iconv/iconv_test.v
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[huly-for-github]

Connected to Huly®: V_0.6-22001

[spytheman]

Thank you.