tools/exec: Implement basic environment overrides.

VAL=xxx... and VAL=$VAL:xxx... are supported; all other syntaxes
will fail with an error message.

When combined with a build/jam patch that will come in a later
commit, this makes it possible to build a large number of targets
using exec as JAMSHELL; including all of libroot. The performance
difference is extremely obvious:

jam -j2 libroot, JAMSHELL=/bin/sh (32-bit Haiku)
real 1m43.571s
user 1m10.961s
sys  1m7.965s

jam -j2 libroot, JAMSHELL=exec
real 1m28.364s
user 0m58.190s
sys  0m57.563s

So that is a savings of 15.21 seconds, or 15% of the build time.
Something that is less I/O bound and more fork-bound (e.g.
linking application catalogs) will almost certainly see
an even bigger performance difference.

Changes to add the necessary JAMSHELL overrides for those
targets which need it, in order to make it possible to
enable usage of "exec" by default, will be coming
over the next few days/weeks...

src/tools/exec.c

@@ -12,6 +12,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <errno.h>
+#include <unistd.h>
 
 
 static void
@@ -66,7 +67,8 @@ main(int argc, const char* argv[])
 	char** args = NULL, *currentArg = NULL;
 	const char* str;
 	int argsLen = 0, argsBufferLen = 0, currentArgLen = 0,
-		currentArgBufferLen = 0, encounteredNewlineAt = -1, pos;
+		currentArgBufferLen = 0, encounteredNewlineAt = -1,
+		modifiedEnvironment = 0, pos;
 
 	if (argc != 2) {
 		printf("usage: %s \"program arg 'arg1' ...\"\n", argv[0]);
@@ -83,7 +85,7 @@ main(int argc, const char* argv[])
 			// We don't support that here, so we need to make sure
 			// that either we have not parsed any arguments yet,
 			// or there are no more arguments pushed after this.
-			if (argsLen == 0 && currentArgLen == 0)
+			if (argsLen == 0 && currentArgLen == 0 && !modifiedEnvironment)
 				break;
 			encounteredNewlineAt = argsLen + 1;
 			// fall through
@@ -97,9 +99,65 @@ main(int argc, const char* argv[])
 				return 1;
 			}
 
+			// the current argument hasn't been terminated, do that now
 			append_char('\0', &currentArg, &currentArgLen,
 				&currentArgBufferLen);
 
+			// handle environs
+			{
+			char* val;
+			if (argsLen == 0 && (val = strstr(currentArg, "=")) != NULL) {
+				const char* dollar;
+				char* newVal = NULL;
+				*val = '\0';
+				val++;
+
+				// handle trivial variable substitution, i.e. VAL=$VAL:...
+				dollar = strstr(val, "$");
+				if (dollar != NULL) {
+					const char* oldVal;
+					int oldValLen, valLen, nameLen;
+
+					if (dollar != val) {
+						puts("exec: environ expansion not at start of "
+							"line unsupported");
+						return 1;
+					}
+					val++; // skip the $
+					valLen = strlen(val);
+					nameLen = strlen(currentArg);
+
+					// if the new value does not start with the environ name
+					// (which is broken by a non-alphanumeric character), bail.
+					if (strncmp(val, currentArg, nameLen) != 0
+							|| isalnum(val[nameLen])) {
+						puts("exec: environ expansion of other variables "
+							"unsupported");
+						return 1;
+					}
+
+					// get the old value and actually do the expansion
+					oldVal = getenv(currentArg);
+					oldValLen = strlen(oldVal);
+					newVal = malloc(valLen + oldValLen + 1);
+					memcpy(newVal, oldVal, oldValLen);
+					memcpy(newVal + oldValLen, val + nameLen, valLen + 1);
+					val = newVal;
+				}
+
+				setenv(currentArg, val, 1);
+				free(newVal);
+				modifiedEnvironment = 1;
+
+				free(currentArg);
+				currentArg = NULL;
+				currentArgLen = 0;
+				currentArgBufferLen = 0;
+				break;
+			}
+			}
+
+			// actually add the argument to the array
 			if ((argsLen + 2) >= argsBufferLen) {
 				args = realloc(args, (argsBufferLen + 8) * sizeof(char*));
 				if (args == NULL) {