jemalloc: Fix test for Linuxbrew
jemalloc: Fix test for Linuxbrew

Specify --std=c99 when compiling the test.

Fixes error:
    'for' loop initial declarations are only allowed in C99 mode

Closes Linuxbrew/homebrew-core#568.

Signed-off-by: Bob W. Hogg <c772a964fd55352a3510e5d535dd9ccc9ac30168@linux.com>

maxim-belkin/homebrew-core,rwhogg/homebrew-core,rwhogg/homebrew-core,Linuxbrew/homebrew-core,LinuxbrewTestBot/homebrew-core,LinuxbrewTestBot/homebrew-core,maxim-belkin/homebrew-core,Linuxbrew/homebrew-core