fix(gcc): explicitly check existence of the commands

When at least one of the commands "cc", "c++", "f77", and "f95" is not
found in the system, on the first load of "gcc" completions,
"command_not_found_handle" hook (typically set by distributions) is
invoked in the middle of completion, breaks the terminal layout by
outputting messages, and eats user inputs.  This commit fixes the
issue by checking the existence of commands before running these
commands.

Refs.
#390
akinomyoga/ble.sh#192
akinomyoga/ble.sh#203

Author: akinomyoga

completions/gcc

@@ -58,25 +58,25 @@ _gcc()
     complete -F _gcc gcc{,-5,-6,-7,-8} g++{,-5,-6,-7,-8} g77 g95 \
         gccgo{,-5,-6,-7,-8} gcj gfortran{,-5,-6,-7,-8} gpc &&
     {
-        if cc --version 2>/dev/null | command grep -q GCC ||
+        if type cc &>/dev/null && cc --version 2>/dev/null | command grep -q GCC ||
             [[ $(_realcommand cc) == *gcc* ]]; then
             complete -F _gcc cc
         else
             complete -F _minimal cc
         fi
-        if c++ --version 2>/dev/null | command grep -q GCC ||
+        if type c++ &>/dev/null && c++ --version 2>/dev/null | command grep -q GCC ||
             [[ $(_realcommand c++) == *g++* ]]; then
             complete -F _gcc c++
         else
             complete -F _minimal c++
         fi
-        if f77 --version 2>/dev/null | command grep -q GCC ||
+        if type f77 &>/dev/null && f77 --version 2>/dev/null | command grep -q GCC ||
             [[ $(_realcommand f77) == *gfortran* ]]; then
             complete -F _gcc f77
         else
             complete -F _minimal f77
         fi
-        if f95 --version 2>/dev/null | command grep -q GCC ||
+        if type f95 &>/dev/null && f95 --version 2>/dev/null | command grep -q GCC ||
             [[ $(_realcommand f95) == *gfortran* ]]; then
             complete -F _gcc f95
         else